shared-memory based stats collector

Hello.

This is intended to provide more stats like the following thread.

https://www.postgresql.org/message-id/20171010.192616.108347483.horiguchi.kyotaro@lab.ntt.co.jp

Most major obstracle for having more items in statistics
collector views comes from the mechanism to share the values
among backends. It is currently using a file. The stats collector
writes a file by triggers from backens then backens reads the
written file. Larger file makes the latency longer and we don't
have a spare bandwidth for additional statistics items.

Nowadays PostgreSQL has dynamic shared hash (dshash) so we can
use this as the main storage of statistics. We can share data
without a stress using this.

A PoC previously posted tried to use "locally copied" dshash but
it doesn't looks fine so I steered to different direction.

With this patch dshash can create a local copy based on dynhash.

This patch consists of tree files.

v1-0001-Give-dshash-ability-to-make-a-local-snapshot.patch

adds dshash to make a local copy backed by dynahash.

v1-0002-Change-stats-collector-to-an-axiliary-process.patch

change the stats collector to be a auxiliary process so that it
can attach dynamic shared memory.

v1-0003-dshash-based-stats-collector.patch

implements shared-memory based stats collector.

I'll put more detailed explanation later.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center