**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
).all()
        
        
        if len(task_instances_to_examine) == 0:
            self.log.debug("No tasks to consider for execution.")
            return executable_tis
        
        task_instance_str = "\n\t".join(
            [repr(x) for x in task_instances_to_examine])
        self.log.info(
            "%s tasks up for execution:\n\t%s", len(task_instances_to_examine),
            task_instance_str
        )
        pool_to_task_instances: DefaultDict[str, List[models.Pool]] = defaultdict(list)
        for task_instance in task_instances_to_examine:
            pool_to_task_instances[task_instance.pool].append(task_instance)
        
        dag_concurrency_map: DefaultDict[str, int]
        task_concurrency_map: DefaultDict[Tuple[str, str], int]
        dag_concurrency_map, task_concurrency_map = self.__get_concurrency_maps(
            states=list(EXECUTION_STATES), session=session)
        num_tasks_in_executor = 0
        
        num_starving_tasks_total = 0
        
        
        
        for pool, task_instances in pool_to_task_instances.items():
            pool_name = pool
            if pool not in pools: