Return the old counter value in isc_stats_increment

Returning the value allows for better high-water tracking without
running into edge cases like the following:

0. The counter is at value X
1. Increment the value (X+1)
2. The value is decreased multiple times in another threads (X+1-Y)
3. Get the value (X+1-Y)
4. Update-if-greater misses the X+1 value which should have been the
   high-water

lib/isc/include/isc/stats.h

@@ -137,10 +137,10 @@ isc_stats_ncounters(isc_stats_t *stats);
  *
  */
 
-void
+isc_statscounter_t
 isc_stats_increment(isc_stats_t *stats, isc_statscounter_t counter);
 /*%<
- * Increment the counter-th counter of stats.
+ * Increment the counter-th counter of stats and return the old value.
  *
  * Requires:
  *\li	'stats' is a valid isc_stats_t.

lib/isc/stats.c

@@ -97,12 +97,12 @@ isc_stats_create(isc_mem_t *mctx, isc_stats_t **statsp, int ncounters) {
 	*statsp = stats;
 }
 
-void
+isc_statscounter_t
 isc_stats_increment(isc_stats_t *stats, isc_statscounter_t counter) {
 	REQUIRE(ISC_STATS_VALID(stats));
 	REQUIRE(counter < stats->ncounters);
 
-	atomic_fetch_add_relaxed(&stats->counters[counter], 1);
+	return (atomic_fetch_add_relaxed(&stats->counters[counter], 1));
 }
 
 void

lib/ns/include/ns/stats.h

@@ -124,7 +124,7 @@ ns_stats_detach(ns_stats_t **statsp);
 void
 ns_stats_create(isc_mem_t *mctx, int ncounters, ns_stats_t **statsp);
 
-void
+isc_statscounter_t
 ns_stats_increment(ns_stats_t *stats, isc_statscounter_t counter);
 
 void

lib/ns/stats.c

@@ -78,11 +78,11 @@ ns_stats_create(isc_mem_t *mctx, int ncounters, ns_stats_t **statsp) {
 /*%
  * Increment/Decrement methods
  */
-void
+isc_statscounter_t
 ns_stats_increment(ns_stats_t *stats, isc_statscounter_t counter) {
 	REQUIRE(NS_STATS_VALID(stats));
 
-	isc_stats_increment(stats->counters, counter);
+	return (isc_stats_increment(stats->counters, counter));
 }
 
 void