Sync latest RPC changes from oslo

Pulls in capped version support and RPC_API_NAMESPACE support for
RpcProxy.

Change-Id: I8334ed62c0ffd16a05b35bbcd4d9352538ab0243
Sync RPC serializer changes from Oslo

Pulls in the serializer object and changes to the dispatcher and
proxy implementations to support it.

Needed for bp/rpc-support-for-objects

Change-Id: I349de94c80a183f244cbb10009b6922996cc4d82
Merge "Sync latest RPC changes from oslo"
Merge "Sync RPC serializer changes from Oslo"