Add base support for rpc API versioning.

Add base support for rpc API versioning.

Part of blueprint versioned-rpc-apis.

This commit includes the base support for versioned RPC APIs.  It
introduces the RpcProxy and RpcDispatcher classes that have common code
for handling versioning on the client and server sides, respectively.

RPC APIs will be converted one at a time using this infrastructure.

Change-Id: I07bd82e9ff60c356123950e466caaffdfce79eba
Reviewed-on: https://review.openstack.org/9901
Reviewed-by: Huang Zhiteng <zhiteng.huang@intel.com>
Reviewed-by: Vish Ishaya <vishvananda@gmail.com>
Approved: John Griffith <john.griffith@solidfire.com>
Tested-by: Jenkins