Add isc_timer_running() function to check status of timer

In the next commit, we need to know whether the timer has been started
or stopped.  Add isc_timer_running() function that returns true if the
timer has been started.

(cherry picked from commit b9e3cd5d2a)

lib/isc/include/isc/timer.h

@@ -156,4 +156,14 @@ isc_timer_destroy(isc_timer_t **timerp);
  *\li	*timerp is NULL.
  */
 
+bool
+isc_timer_running(isc_timer_t *timer);
+/*%<
+ * Return true if the timer has been started.
+ *
+ * Requires:
+ *
+ *\li	'timer' is a valid timer*
+ */
+
 ISC_LANG_ENDDECLS

lib/isc/timer.c

@@ -16,6 +16,7 @@
 #include <stdbool.h>
 
 #include <isc/async.h>
+#include <isc/atomic.h>
 #include <isc/condition.h>
 #include <isc/heap.h>
 #include <isc/job.h>
@@ -196,3 +197,10 @@ isc_timer_async_destroy(isc_timer_t **timerp) {
 	isc_timer_stop(timer);
 	isc_async_run(timer->loop, timer_destroy, timer);
 }
+
+bool
+isc_timer_running(isc_timer_t *timer) {
+	REQUIRE(VALID_TIMER(timer));
+
+	return atomic_load_acquire(&timer->running);
+}