Implement pillar adding and removal to collections in the IntegrityService

This involves two scenarios:
A new pillar has been added to the list of pillars in settings and is therefore missing from the IntegrityService database.
A pillar has been removed from the list of pillars in the settings.

It should be possible to add another pillar to the existing list of pillars in the database, though it would require, that an entry for all files in the fileinfo table is created for this new pillar.

It is uncertain how to handle the case, when a pillar has been removed. It must be discussed with the architects.