Replace the pass-through LocalTransport with LocalRPCTransport.

Summary:
Remove some redundant shims by borrowing the net/rpc client interface
more directly.  rpc.Call is a suitable container for a request/response
pair even when net/rpc is not used as the underlying transport.

Test Plan: go test

Reviewers: spencerkimball

Reviewed By: spencerkimball

Differential Revision: http://phabricator.andybons.com/D22