Precompiled 2025-11-22-73af2f4

Stop using a FollowerRetentionManager in database bootstrap (#43443)

Using FollowerRetentionManager needlessly makes database initialization (`DatabaseSnapshot::load`)
and retention workers slower because they repeatedly query persistence globals to get the
current snapshot timestamps, even though we hold the lease and are the only writer.

We can instead make everything use LeaderRetentionManager by splitting up its initialization into
two parts - one that reads the initial snapshot bounds & returns a usable RetentionValidator,
then a second part that actually launches the retention workers.

GitOrigin-RevId: cb8aef3f6349eafdd0efd73115b6bffcf242689e