additional request API tweaks and testing 

In the following series, some convenience functions are added
* `flux_msg_has_payload()` boolean test for presence of payload
* `flux_response_decode()` decode a response with no payload expected

One function `flux_response_recv()` which was on the fence was moved into the deprecated section of request.h.  Inline doc for flux_response_putmsg() was expanded a bit per Mark's request.

Some tests were added to further exercise request code, including `flux_response_putmsg()`.