Filter creation with cron job

[georgmaisser]

The creation of the filter is a very expensive operation, as it needs to fetch all possible results to create the filter. Any change in a table will invalidate the filter and will make a new creation necessary.

We should implement a logic to recreate the filter tight after the table cache was purged, independent of an actual user requesting the specific table.

The way to go would be:

• [ ] When calling a filter creating sql, save this sql in a separate cache
• [ ] When purging cache, get this sql from the separate cache
• [ ] Add ad hoc task to be run immediately with the command to execute sql and, in the process, recreate filter
• [ ] To make sure we don't waste resources, there must be a short (or even better a variable and potentially increasing) delay to make sure we don't get hit by a period of very fast turning caches (eg: uploading a lot of new records where after every record the cache is purged)

This will not take care of a "purge all caches" situation, but that is not what usually happens. We just want to improve performance for users on fairly (but not very) often turning table caches.

[georgmaisser]

Because of the success of task #46, we close this, it does not seem to be necessary.