librbd: fix parent cache races and error handling

librbd: fix parent cache races and error handling

If the plugin fails to connect to the daemon at start-up it will
crash the process due to a resource deadlock exception being
thrown as the client is destroyed. Additionally, librbd will support
concurrent IO thread processing in the future so the client needs
to be protected by a lock.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>
(cherry picked from commit 74a139d2ceb068fd5c77884e2a7be0359a3300c9)

Conflicts:
	src/librbd/cache/ParentCacheObjectDispatch.cc:
          register_dispatch->register_object_dispatch