ABI/API break: pas in TID_RESP * to handler

ABI/API break: pas in TID_RESP * to handler

Previously, we passed in TID_RESP ** to the request handler.  However
the request handlers assumed that the response was allocated.  We
don't want responses allocated in the handler, so make it a single
pointer.

note that the existing handler interface is probably inappropriate for
an event-loop-based trust router.