pgsql: Add support for zstd with compression of full-page writes in WAL

Add support for zstd with compression of full-page writes in WAL

wal_compression gains a new value, "zstd", to allow the compression of
full-page images using the compression method of the same name.

Compression is done using the default level recommended by the library,
as of ZSTD_CLEVEL_DEFAULT = 3.  Some benchmarking has shown that it
could make sense to use a level lower for the FPI compression, like 1 or
2, as the compression rate did not change much with a bit less CPU
consumed, but any tests done would only cover few scenarios so it is
hard to come to a clear conclusion.  Anyway, there is no reason to not
use the default level instead, which is the level recommended by the
library so it should be fine for most cases.

zstd outclasses easily pglz, and is better than LZ4 where one wants to
have more compression at the cost of extra CPU but both are good enough
in their own scenarios, so the choice between one or the other of these
comes to a study of the workload patterns and the schema involved,
mainly.

This commit relies heavily on 4035cd5, that reshaped the code creating
and restoring full-page writes to be aware of the compression type,
making this integration straight-forward.

This patch borrows some early work from Andrey Borodin, though the patch
got a complete rewrite.

Author: Justin Pryzby
Discussion: https://postgr.es/m/20220222231948.GJ9008@telsasoft.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e9537321a74a2b062c8f7a452314b4570913f780

Modified Files
--------------
doc/src/sgml/config.sgml                      | 11 +++++----
doc/src/sgml/installation.sgml                |  8 +++++++
src/backend/access/transam/xloginsert.c       | 32 ++++++++++++++++++++++++++-
src/backend/access/transam/xlogreader.c       | 20 +++++++++++++++++
src/backend/utils/misc/guc.c                  |  3 +++
src/backend/utils/misc/postgresql.conf.sample |  2 +-
src/bin/pg_waldump/pg_waldump.c               |  2 ++
src/include/access/xlog.h                     |  3 ++-
src/include/access/xlogrecord.h               |  5 ++++-
9 files changed, 78 insertions(+), 8 deletions(-)