Bug 2526: Race condition may cause missing routes in RPC BucketStore

Changed the RpcRegistry class to derive from BucketStore instead of
creating the BucketStore as a separate actor. This was done because the
copy-on-write update operations in the BucketStore need to be done
atomically so the RpcRegistry needs direct access to the BucketStore in
order to do this safely and efficiently. The RpcRegistry handles the
sematics of the Bucket data - this keeps the BucketStore data agnostic.

Change-Id: Ief96c28775b729f459d324971403222e5a578029
Signed-off-by: tpantelis <tpanteli@brocade.com>