The original coding required that PQpipelineSync had been called before
the first call to PQgetResult, and failure to do that would result in an
unexpected NULL result being returned.  Fix by setting the right state
when a query is sent, rather than leaving it unchanged and having
PQpipelineSync apply the necessary state change.

A new test case to verify the behavior is added, which relies on the new
PQsendFlushRequest() function added by commit a7192326c74d.

Backpatch to 14, where pipeline mode was added.
Reported-by: Boris Kolpackov <boris@codesynthesis.com>
Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/boris.20210616110321@codesynthesis.com