**Environment**:

Breeze Sqlite version


**What happened**:

With the current master version of Airflow there is a problem with locking the database when running default Airflow configuration in Sqlite in Breeze.

## How to reproduce


```./breeze start-airflow --load-example-dags```

Start running/clicking on DAGS, You will soon start getting errors: 


```
he above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 2447, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 1952, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 1821, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/local/lib/python3.6/site-packages/flask/_compat.py", line 39, in reraise
    raise value
  File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 1950, in full_dispatch_request
    rv = self.dispatch_request()
  File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 1936, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/opt/airflow/airflow/www/decorators.py", line 117, in wrapper
    return f(self, *args, **kwargs)
  File "/usr/local/lib/python3.6/site-packages/flask_appbuilder/security/decorators.py", line 109, in wraps
    return f(self, *args, **kwargs)
  File "/opt/airflow/airflow/www/decorators.py", line 98, in view_func
    return f(*args, **kwargs)
  File "/opt/airflow/airflow/www/decorators.py", line 59, in wrapper
    session.add(log)
  File "/usr/local/lib/python3.6/contextlib.py", line 88, in __exit__
    next(self.gen)
  File "/opt/airflow/airflow/utils/session.py", line 31, in create_session
    session.commit()
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/orm/session.py", line 1042, in commit
    self.transaction.commit()
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/orm/session.py", line 504, in commit
    self._prepare_impl()
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/orm/session.py", line 483, in _prepare_impl
    self.session.flush()
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/orm/session.py", line 2523, in flush
    self._flush(objects)
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/orm/session.py", line 2664, in _flush
    transaction.rollback(_capture_exception=True)
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/util/langhelpers.py", line 69, in __exit__
    exc_value, with_traceback=exc_tb,
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/util/compat.py", line 182, in raise_
    raise exception
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/orm/session.py", line 2624, in _flush
    flush_context.execute()
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/orm/unitofwork.py", line 422, in execute
    rec.execute(self)
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/orm/unitofwork.py", line 589, in execute
    uow,
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/orm/persistence.py", line 245, in save_obj
    insert,
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/orm/persistence.py", line 1136, in _emit_insert_statements
    statement, params
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/engine/base.py", line 1011, in execute
    return meth(self, multiparams, params)
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/sql/elements.py", line 298, in _execute_on_connection
    return connection._execute_clauseelement(self, multiparams, params)
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/engine/base.py", line 1130, in _execute_clauseelement
    distilled_params,
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/engine/base.py", line 1317, in _execute_context
    e, statement, parameters, cursor, context
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/engine/base.py", line 1511, in _handle_dbapi_exception
    sqlalchemy_exception, with_traceback=exc_info[2], from_=e
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/util/compat.py", line 182, in raise_
    raise exception
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/engine/base.py", line 1277, in _execute_context
    cursor, statement, parameters, context
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/engine/default.py", line 593, in do_execute
    cursor.execute(statement, parameters)
sqlalchemy.exc.OperationalError: (sqlite3.OperationalError) database is locked
[SQL: INSERT INTO log (dttm, dag_id, task_id, event, execution_date, owner, extra) VALUES (?, ?, ?, ?, ?, ?, ?)]
[parameters: ('2020-10-23 16:35:08.015990', 'example_branch_operator', None, 'tree', None, 'admin', "[('dag_id', 'example_branch_operator')]")]                                                                                                                                        
(Background on this error at: http://sqlalche.me/e/13/e3q8)
```




**Anything else we need to know**:

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airflow/jobs/scheduler_job.py
self.log.warning(
                    "Tasks using non-existent pool '%s' will not be scheduled",
                    pool
                )
                continue
            open_slots = pools[pool]["open"]
            num_ready = len(task_instances)
            self.log.info(
                "Figuring out tasks to run in Pool(name=%s) with %s open slots "
                "and %s task instances ready to be queued",
                pool, open_slots, num_ready
            )
            priority_sorted_task_instances = sorted(
                task_instances, key=lambda ti: (-ti.priority_weight, ti.execution_date))
            num_starving_tasks = 0
            for current_index, task_instance in enumerate(priority_sorted_task_instances):
                if open_slots <= 0:
                    self.log.info(
                        "Not scheduling since there are %s open slots in pool %s",
                        open_slots, pool
                    )
                    
                    num_unhandled = len(priority_sorted_task_instances) - current_index
                    num_starving_tasks += num_unhandled
                    num_starving_tasks_total += num_unhandled
                    break
                
                
                dag_id = task_instance.dag_id
                current_dag_concurrency = dag_concurrency_map[dag_id]