ReplicatedPG,osd_types: move rw tracking from its own map to ObjectContext

We also modify recovering to hold a reference to the recovering obc
in order to ensure that our backfill_read_lock doesn't outlive the
obc.

Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG: have make_writeable adjust backfill_pos

If we are writing to backfill_pos and create a clone, we end
up failing to send the transaction creating the clone to the
backfill peer.  This is fine as long as we end up backfilling
the clone.  To that end, we simply add the clone to
backfill_info and adjust backfill_pos accordingly.  This is less
brittle than the waiting_for_backfill_pos mechanism since it
works even if we wait between that check and issuing the repop,
which can happen for copy_from.

Signed-off-by: Samuel Just <sam.just@inktank.com>
PG,ReplicatedPG: remove the waiting_for_backfill_peer mechanism

See previous patch.

Signed-off-by: Samuel Just <sam.just@inktank.com>
PG: call on_flushed on FlushEvt