ReplicatedPG: replace map iterators with SharedPtrRegistry::get_next

ReplicatedPG: replace map iterators with SharedPtrRegistry::get_next

SharedPtrRegistry does not provide an iterator equivalent to

    map<hobject_t, ObjectContext*>::iterator i

It is replaced with a thread safe get_next method roughly used
as follows:

    pair<hobject_t, ObjectContextRef> i;
    while (object_contexts.get_next(i.first, &i))

All occurences of the iterator are replaced with get_next style
traversal.

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

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