This is useful for applications that want to micromanage the rpc service.
Moved part of ServeCodec into a new readRequest function.
Renamed existing readRequest to readRequestHeader, and reordered
its parameters to align with the new readRequest and service.call.

R=golang-dev, r, rsc, sougou
CC=golang-dev, msolomon
https://golang.org/cl/4889043