Implement executor queue for batch changes executor

Created by: eseliger

The very first PR towards server-side batch changes! This PR adds a queue and a database table to queue from for batch_spec_executions, as well as a resetter (as it is essentially a super fancy dbworker).

Review commit-by-commit for small digestible changes.

There is potentially a chance to get rid of some duplicative code between codeintel and batches, but so far we don't really know where the journey will lead us, so I don't want to do too many optimizations right now.

@efritz Can you please review if I correctly register all the required things (queue, resetter, (????)) and use the executor-queue as expected? Also, is every customer running an instance of the executor-queue?

This PR does NOT add a separate executor that pulls from this queue yet, that will follow in a separate PR.