pgsql: Add PQresultMemorySize function to report allocated size of aPG

Add PQresultMemorySize function to report allocated size of a PGresult.

This number can be useful for application memory management, and the
overhead to track it seems pretty trivial.

Lars Kanis, reviewed by Pavel Stehule, some mods by me

Discussion: https://postgr.es/m/fa16a288-9685-14f2-97c8-b8ac84365a4f@greiz-reinsdorf.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2970afa6cf1057107c998bf3cdd1fbf6dc78cf6c

Modified Files
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doc/src/sgml/libpq.sgml          | 34 ++++++++++++++++++++++++++++++++++
src/interfaces/libpq/exports.txt |  1 +
src/interfaces/libpq/fe-exec.c   | 38 ++++++++++++++++++++++++++++++++------
src/interfaces/libpq/libpq-fe.h  |  1 +
src/interfaces/libpq/libpq-int.h |  2 ++
5 files changed, 70 insertions(+), 6 deletions(-)