This adds an explicit queuing mechanis for reindexing. The rake task
works as follows:

1. Trigger async index creation if necessary (still a hack)
1. Cleanup leftovers from previous run
1. Consume from the queue if non-empty
1. Execute heuristic to choose indexes for regular reindexing

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/330922

Changelog: added