Add a function that removes unwanted items from the database at regular intervals

[Nicklas Nordborg]

Unwanted items are:

• Stray any-to-any links (see #1340)
• Unused item and project keys

The cleanup should be executed at regular intervals (possible configurable). It should be possible to disable this function since we don't want to execute it from job agents.

[Nicklas Nordborg]

(In [5023]) Fixes #1351: Add a function that removes unwanted items from the database at regular intervals

[Nicklas Nordborg]

(In [5024]) References #1351: Add a function that removes unwanted items from the database at regular intervals

Fixed problems with remaining garbage when TestAll was executed multiple times.