These assignments caused style warnings (flake8).

The current target length is 99 characters.

Thanks, flake8.

The exceptions to be handled should be specified, otherwise even a
"NameError" would be caught.

Thanks, flake8.

Thanks, flake8.

Refactoring and fixes related to end of timeslot and channels

See merge request aura/engine!12

This solves issues with concurrent Liquidsoap operations.

This temporarly solves:
- engine-core#5

This relates to:
- engine#65
- engine#76

Rework database session handling

See merge request aura/engine!11

Currently the database session is created during application startup and
is used in all threads.  This is problematic, since the session is not
thread-safe.
Now we use the request-based database session provided by
flask-sqlachemy instead.

But this does not really solve the problem of using the same session in
different threads, since the threads are started within a request.

The introduction of short-lived sessions caused issues with functions,
which return database objects.  Such objects are not accessible (due to
lazy loading), after the session is closed.

Instead we should either move the session scope the respective callers
or use a request-based session handling (e.g. the one provided by
flask-sqlalchemy).

This reverts commit 0b594e50 and 70ffd1a0.

See: #93