ReplicatedPG: ObjectContext * becomes ObjectContextRef

ReplicatedPG: ObjectContext * becomes ObjectContextRef

The map of hobject_t to ObjectContext is made a
SharedPtrRegistry owned by ReplicatedPG

    -  map<hobject_t, ObjectContext*> object_contexts;
    +  SharedPtrRegistry<hobject_t, ObjectContext> object_contexts;

All ObjectContext pointers are changed into ObjectContextRef, i.e.
shared_ptr.

In Watch.h std::tr1::shared_ptr<ObjectContext> is used instead
of ObjectContextRef because Watch.h is included before it is
defined.

http://tracker.ceph.com/issues/5510 refs #5510

Signed-off-by: Loic Dachary <loic@dachary.org>