A restore wipes and rehydrates the uploads under a swapped database, but it
never took the storage job slot, so a reclaim pass could start or carry on
beside it, and a delete already past the generation check could land on an
object the restore had just put back. BackupService now marks a restore as
running on StorageJobsService and waits for a pass in flight to let go; a
pass refuses to start while a restore runs and stops at its next check when
one begins. Backfills and migrations are not affected. The service test also
reads its fake source sets through a helper instead of non-null assertions.
Maurice
19e4f0dadf