Fix a deadlock issue when using with-batch-calls over a synchronous transport.
Fix a deadlock issue when using with-batch-calls over a synchronous transport.

INVOKE-RPC-METHOD was returning RPC-CALL-PROMISE unconditionally, which
triggers a read for synchronous transports. The fix is to return a stub
promise that WITH-BATCH-CALLS will complete with the normal promise after
submitting the request.

mtstickney/cl-mtgnet