Add index to Challenge.expiration column

[plettich]

The Challenge table does not have an index on the expiration column but it is used throughout the code to find expired challenges. We should add an index to the column.
Additionally we call cleanup_challenges() during a request which tries to delete all expired challenges:

privacyidea/privacyidea/models.py

Line 1490 in 5c16530

def cleanup_challenges():

This probably needs to lock the complete table (due to the expiration < <date> condition) and could lead to deadlocks.
A request should only delete it's own challenge (either because it is answered or because it expired) but not other challenges.
The challenge table can be cleared by an external script or a task runner.

[nilsbehlen]

use cleanup_challenges only for the own successful challenges. Expired challenges should be removed externally -> cron.

[nilsbehlen]

• remove challengejanitor from code.
• add external janitor
• add index