Rather than employ the typeurl package, we now generate code to
correctly allocate the incoming types from the caller. As a side-effect
of this activity, the services definitions have been split out into a
separate type that handles the full resolution and dispatch of the
method, incuding correctly mapping the RPC status.
This work is a pre-cursor to larger protocol change that will allow us
to handle multiple, concurrent requests.
Signed-off-by: Stephen J Day <stephen.day@docker.com>