pgsql: Add show_data option to pg_get_wal_block_info.

Add show_data option to pg_get_wal_block_info.

Allow users to opt out of returning FPI data and block data from
pg_get_wal_block_info as an optimization. Testing has shown that this
can make function execution over twice as fast in some cases.

When pg_get_wal_block_info is called with "show_data := false", it
always returns NULL values for its block_data and block_fpi_data bytea
output parameters. Nothing else changes. In particular, the function
will still return the usual per-block summary of block data/FPI space
overhead. Use of "show_data := false" is therefore feasible with all
queries that don't specifically require these raw binary strings.

Follow-up to recent work in commit 122376f0. There still hasn't been a
stable release with the pg_get_wal_block_info function, so no bump in
the pg_walinspect extension version.

Per suggestion from Melanie Plageman.

Author: Peter Geoghegan <pg(at)bowt(dot)ie>
Discussion: https://postgr.es/m/CAAKRu_bJvbcYBRj2cN6G2xV7B7-Ja+pjTO1nEnEhRR8OXYiABA@mail.gmail.com
Discussion: https://postgr.es/m/CAH2-Wzm9shOkEDM10_+qOZkRSQhKVxwBFiehH6EHWQQRd_rDPw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/df4f3ab51730a4cdddfead0b264d394ee4925723

Modified Files
--------------
contrib/pg_walinspect/expected/oldextversions.out | 8 ++++----
contrib/pg_walinspect/expected/pg_walinspect.out | 10 +++++-----
contrib/pg_walinspect/pg_walinspect--1.0--1.1.sql | 5 +++--
contrib/pg_walinspect/pg_walinspect.c | 13 ++++++++-----
contrib/pg_walinspect/sql/pg_walinspect.sql | 10 +++++-----
doc/src/sgml/pgwalinspect.sgml | 17 ++++++++++++++---
6 files changed, 39 insertions(+), 24 deletions(-)