libflux: clarify desc of flux_response_putmsg
libflux: add flux_msg_has_payload()
libflux: add flux_response_decode()

Need function for decoding messages with no payload.
libflux: deprecate flux_response_recv()
test: expand request testing

Add test for receiving a large batch of sequenced responses.
Add test to ensure flux_response_putmsg() respects message ordering.