**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
super().__init__()
        self.dag_ids = dag_ids
        self._log = log
    @provide_session
    def manage_slas(self, dag: DAG, session: Session = None) -> None:
        """
        Finding all tasks that have SLAs defined, and sending alert emails
        where needed. New SLA misses are also recorded in the database.
        We are assuming that the scheduler runs often, so we only check for
        tasks that should have succeeded in the past hour.
        """
        if not any(isinstance(ti.sla, timedelta) for ti in dag.tasks):
            self.log.info("Skipping SLA check for %s because no tasks in DAG have SLAs", dag)
            return
        qry = (
            session
            .query(
                TI.task_id,
                func.max(TI.execution_date).label('max_ti')
            )
            .with_hint(TI, 'USE INDEX (PRIMARY)', dialect_name='mysql')
            .filter(TI.dag_id == dag.dag_id)