pgsql: Avoid spamming the client with multiple ParameterStatus messages

Avoid spamming the client with multiple ParameterStatus messages.

Up to now, we sent a ParameterStatus message to the client immediately
upon any change in the active value of any GUC_REPORT variable. This
was only barely okay when the feature was designed; now that we have
things like function SET clauses, there are very plausible use-cases
where a GUC_REPORT variable might change many times within a query
--- and even end up back at its original value, perhaps. Fortunately
most of our GUC_REPORT variables are unlikely to be changed often;
but there are proposals in play to enlarge that set, or even make it
user-configurable.

Hence, let's fix things to not generate more than one ParameterStatus
message per variable per query, and to not send any message at all
unless the end-of-query value is different from what we last reported.

Discussion: https://postgr.es/m/5708.1601145259@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2432b1a04087edc2fd9536c7c9aa4ca03fd1b363

Modified Files
--------------
src/backend/tcop/postgres.c | 3 ++
src/backend/utils/misc/guc.c | 78 ++++++++++++++++++++++++++++++++++++++----
src/include/utils/guc.h | 1 +
src/include/utils/guc_tables.h | 5 ++-
4 files changed, 80 insertions(+), 7 deletions(-)