RequestCorrelator: request-ID is sent in request and header

When invoking a sync RPC, a request is identified by a request ID (req_id, a long). This is sent in both the header and the request itself, so we can remove one of them.
Also, in Header, we can drop field rsp_expected: this can be replaced by setting req_id to -1.
Summary: we can drop a long (req_id) field in Request and a boolean (rsp_expected in Header) from each RPC.