Enable the pageserver to recover from data corruption events by
implementing a feature to re-apply historic WAL records in parallel to the already
occurring WAL replay.
The feature is outside of the user-visible backup and history story, and
only
serves as a second-level backup for the case that there is a bug in the
pageservers that corrupted the served pages.
The RFC proposes the addition of two new features:
* recover a broken branch from WAL (downtime is allowed)
* a test recovery system to recover random branches to make sure
recovery works