Moved rpc_compat.py code back into rpc.py

Most of this code will probably stay with us for quite some time, so
let's make things easier and consider them as our way of doing RPC.

blueprint oslo-messaging

Change-Id: Iaf353b23f9c54b82d1e02a6bd5a5960cec827c88
Merge "Moved rpc_compat.py code back into rpc.py"