pgsql: Add EXPLAIN (MEMORY) to report planner memory consumption

Add EXPLAIN (MEMORY) to report planner memory consumption

This adds a new "Memory:" line under the "Planning:" group (which
currently only has "Buffers:") when the MEMORY option is specified.

In order to make the reporting reasonably accurate, we create a separate
memory context for planner activities, to be used only when this option
is given. The total amount of memory allocated by that context is
reported as "allocated"; we subtract memory in the context's freelists
from that and report that result as "used". We use
MemoryContextStatsInternal() to obtain the quantities.

The code structure to show buffer usage during planning was not in
amazing shape, so I (Álvaro) modified the patch a bit to clean that up
in passing.

Author: Ashutosh Bapat
Reviewed-by: David Rowley, Andrey Lepikhov, Jian He, Andy Fan
Discussion: https://www.postgresql.org/message-id/CAExHW5sZA=5LJ_ZPpRO-w09ck8z9p7eaYAqq3Ks9GDfhrxeWBw@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/5de890e3610d5a12cdaea36413d967cf5c544e20

Modified Files
--------------
contrib/auto_explain/auto_explain.c | 2 +
doc/src/sgml/ref/explain.sgml | 14 ++++
src/backend/commands/explain.c | 152 ++++++++++++++++++++++++++++------
src/backend/commands/prepare.c | 22 ++++-
src/backend/utils/mmgr/mcxt.c | 13 +++
src/include/commands/explain.h | 4 +-
src/include/utils/memutils.h | 2 +
src/test/regress/expected/explain.out | 76 +++++++++++++++++
src/test/regress/sql/explain.sql | 8 ++
9 files changed, 265 insertions(+), 28 deletions(-)