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/include/utils/guc_tables.h

@@ -161,6 +161,8 @@ struct config_generic
 	GucContext	reset_scontext; /* context that set the reset value */
 	GucStack   *stack;			/* stacked prior values */
 	void	   *extra;			/* "extra" pointer for current actual value */
+	char	   *last_reported;	/* if variable is GUC_REPORT, value last sent
+								 * to client (NULL if not yet sent) */
 	char	   *sourcefile;		/* file current setting is from (NULL if not
 								 * set in config file) */
 	int			sourceline;		/* line in source file */
@@ -172,7 +174,8 @@ struct config_generic
  * Caution: the GUC_IS_IN_FILE bit is transient state for ProcessConfigFile.
  * Do not assume that its value represents useful information elsewhere.
  */
-#define GUC_PENDING_RESTART 0x0002
+#define GUC_PENDING_RESTART 0x0002	/* changed value cannot be applied yet */
+#define GUC_NEEDS_REPORT	0x0004	/* new value must be reported to client */
 
 
 /* GUC records for specific variable types */