Add PQsendFlushRequest to libpq

Add PQsendFlushRequest to libpq

This new libpq function allows the application to send an 'H' message,
which instructs the server to flush its outgoing buffer.

This hasn't been needed so far because the Sync message already requests
a buffer; and I failed to realize that this was needed in pipeline mode
because PQpipelineSync also causes the buffer to be flushed.  However,
sometimes it is useful to request a flush without establishing a
synchronization point.

Backpatch to 14, where pipeline mode was introduced in libpq.

Reported-by: Boris Kolpackov <boris@codesynthesis.com>
Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/202106252350.t76x73nt643j@alvherre.pgsql