Subject: Call pqPipelineFlush from PQsendFlushRequest

From: Jelte Fennema-Nio <[email-masked]>
Date: Tue, 7 Nov 2023 10:38:04 +0100

In pipeline mode after queuing a message to be sent we would flush the
buffer if the size of the buffer passed some threshold. The only message
type that we didn't do that for was the Flush message. This addresses
that oversight.

I noticed this discrepancy while reviewing the
PQsendSyncMessage/PQpipelinePutSync patchset.