ReplicatedPG: implement the RWTracker mechanisms for backfill read locking

ReplicatedPG: implement the RWTracker mechanisms for backfill read locking

We want backfill to take read locks on the objects it's pushing. Add
a get_backfill_read(hobject_t) function, a corresponding drop_backfill_read(),
and a backfill_waiting_on_read member in ObjState. Check that member when
getting a write lock, and in put_write(). Tell callers to requeue the recovery
if necessary, and clean up the backfill block when its read lock is dropped.

Signed-off-by: Greg Farnum <greg@inktank.com>