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

refactor: use short-lived sessions when accessing the database

Closes #75

See merge request aura/engine!9

feat(database): add support for sqlite database

See merge request aura/engine!5

docs: use local Debian packages for bare-metal deployment

See merge request aura/engine!6

# Conflicts:
#   docs/bare-metal-installation.md

Grammar and other documentation issues

See merge request aura/engine!7

# Conflicts:
#   src/engine.py
#   src/mixer.py

fix(heartbeat-monitor): port the "PyHeartBeat" script to Python3

See merge request aura/engine!8

docs: fix spelling issues

See merge request aura/engine!4

Improve graceful shutdown. #88

See merge request aura/engine!10