Decouple post-processing from requests for Notifications

[uklotzde]

Modifying requests (PUT/POST/DELETE) often require to execute post-processing actions like re-indexing some entries or sending subscription e-mails. Currently those actions are executed within the request context.

The request should succeed as soon as the actual operation has completed successfully. Required follow-up actions need to be scheduled within the database transaction, but must not be executed immediately -> event log table(s). The actual execution is performed later by independent tasks that are triggered on-demand and/or periodically.

[wellemut]

If we build the subscription process completly on the "recently-changed"-API, there is not post precessing for notifications at all anymore:

Checkout: #258 (comment)