shm_mq_sendv: Fix flushing bug when receiver not yet attached.

shm_mq_sendv: Fix flushing bug when receiver not yet attached.

With the old logic, when the reciever had not yet attached, we would
never call shm_mq_inc_bytes_written(), even if force_flush = true
was specified. That could result in a situation where data that the
sender believes it has sent is never received.

Along the way, remove a useless function prototype for a nonexistent
function from shm_mq.h.

Commit 46846433a03dff4f2e08c8a161e54a842da360d6 introduced these
problems.

Pavan Deolasee, with a few changes by me.

Discussion: https://postgr.es/m/CABOikdPkwtLLCTnzzmpSMXo3QZa2yXq0J7Q61ssdLFAJYrOVvQ@mail.gmail.com