This adds 2 new config options: `job_entry_func` and `job_exit_func`.
These functions are executed before and after every single job
execution. The rationale behind this is before and after each request
Django cleans up stale database connections, so Django jobs need some
way of running this same setup/cleanup functions.
https://github.com/django/django/blob/master/django/db/__init__.py#L57
```
# Register an event to reset transaction state and close connections past
# their lifetime.
def close_old_connections(**kwargs):
for conn in connections.all():
conn.close_if_unusable_or_obsolete()
signals.request_started.connect(close_old_connections)
signals.request_finished.connect(close_old_connections)
```
fixes: #41