Add a new indexer that combines all metadata-related indexers in a single task.
Scheduling a task at the end of each origin-head and revision-metadata task was way too slow (approx. 5 seconds, see swh/infra/sysadm-environment#1492 (closed)).
This Diff adds a new indexer that calls the three other indexers without going through the scheduler, which is a lot faster and requires less code.
Further Diffs will clean up the code that is now useless.
Test Plan
tox + docker
Migrated from D986 (view on Phabricator)