**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
:param parent_channel: the parent end of the channel to close in the child
        :type result_channel: multiprocessing.Connection
        :param file_path: the file to process
        :type file_path: str
        :param pickle_dags: whether to pickle the DAGs found in the file and
            save them to the DB
        :type pickle_dags: bool
        :param dag_ids: if specified, only examine DAG ID's that are
            in this list
        :type dag_ids: list[str]
        :param thread_name: the name to use for the process that is launched
        :type thread_name: str
        :param callback_requests: failure callback to execute
        :type callback_requests: List[airflow.utils.callback_requests.CallbackRequest]
        :return: the process that was launched
        :rtype: multiprocessing.Process
        """
        
        log: logging.Logger = logging.getLogger("airflow.processor")
        
        
        log.info("Closing parent pipe")
        parent_channel.close()
        del parent_channel
        set_context(log, file_path)
        setproctitle("airflow scheduler - DagFileProcessor {}".format(file_path))
        try:
            
            with ExitStack() as exit_stack:
                exit_stack.enter_context(redirect_stdout(StreamLogWriter(log, logging.INFO)))