PG-286: Small performance improvements.

pgss_ExecutorEnd: Avoid unnecessary memset(plan_info, 0, ...).
We only use this object if the condition below is true, in which case we
already initialize all the fields in the object, also we now store the
plan string length (plan_info.plan_len) to avoid calling strlen on it
again later:
if (queryDesc->operation == CMD_SELECT && PGSM_QUERY_PLAN) {
... here we initialize plan_info

If the condition is false, then we pass a NULL PlanInfo* to the
pgss_store to avoid more unnecessary processing.

pgss_planner_hook: Similar, avoid memset(plan_info, 0, ...) this object
is not used here, so we pass NULL to pgss_store.

pg_get_application_name: Remove call to strlen, snprintf already give us
the calculated string length, so we just return it.

pg_get_client_addr: Cache localhost, avoid calling
ntohl(inet_addr("127.0.0.1")) all the time.

pgss_update_entry: Make use of PlanInfo->plan_len, avoiding a call to
strlen again.

intarray_get_datum: Init the string by setting the first byte to '\0'.

pg_stat_monitor.h

@@ -189,6 +189,7 @@ typedef struct PlanInfo
 {
 	uint64		planid;						/* plan identifier */
 	char 		plan_text[PLAN_TEXT_LEN];	/* plan text */
+	size_t		plan_len;                   /* strlen(plan_text) */
 } PlanInfo;
 
 typedef struct pgssHashKey