PATCH: Batch/pipelining support for libpq

Hi all

Following on from the foreign table batch inserts thread[1], here's a patch
to add support for pipelining queries into asynchronous batches in libpq.

Attached, and also available at
https://github.com/2ndQuadrant/postgres/tree/dev/libpq-async-batch (subject
to rebasing and force pushes).

It's cleaned up over the draft I posted on that thread and has error
recovery implemented. I've written and included the SGML docs for it. The
test program is now pretty comprehensive, more so than for anything else in
libpq anyway. I'll submit it to the next CF as a 9.7/10.0 candidate.

I'm measuring 300x (not %) performance improvements doing batches on
servers over the Internet, so this seems pretty worthwhile. It turned out
to be way less invasive than I expected too.

(I intentionally didn't add any way for clients to annotate each work-item
in a batch with their own private data. I think that'd be really useful and
would make implementing clients easier, but should be a separate patch).

This should be very useful for optimising FDWs, Postgres-XC, etc.

[1]
http://www.postgresql.org/message-id/CAMsr+YFgDUiJ37DEfPRk8WDBuZ58psdAYJd8iNFSaGxtw=wU3g@mail.gmail.com

--
Craig Ringer http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services