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

src/backend/tcop/postgres.c

@@ -4229,6 +4229,9 @@ PostgresMain(int argc, char *argv[],
 				pgstat_report_activity(STATE_IDLE, NULL);
 			}
 
+			/* Report any recently-changed GUC options */
+			ReportChangedGUCOptions();
+
 			ReadyForQuery(whereToSendOutput);
 			send_ready_for_query = false;
 		}