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

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.

ReplicatedPG::op_applied no longer clears repop->obc since we need
it to live until the op is finally cleaned up.  This is fine since
repop->obc is now an ObjectContextRef and can clean itself up.

Signed-off-by: Samuel Just <sam.just@inktank.com>