When there are no broadcasters and no known subscriptions, only check…

… for updated events once per minute

This will reduce table scans on diagnostic_event_subscription when reaper is distributed and diagnostics not otherwise used/subscribed to.
Based off findings from #896

src/server/src/main/java/io/cassandrareaper/service/DiagEventSubscriptionService.java

@@ -84,6 +84,7 @@ public final class DiagEventSubscriptionService {
   private final AppContext context;
   private final HttpClient httpClient;
   private final ScheduledExecutorService scheduler;
+  private final AtomicLong lastUpdateCheck = new AtomicLong(0);
 
   private Set<DiagEventSubscription> subsAlwaysActive;
 
@@ -135,7 +136,13 @@ public void subscribe(DiagEventSubscription sub, EventOutput eventOutput, String
 
   private synchronized void updateEnabledEvents() {
     if (context.isDistributed.get()) {
-      updateEnabledEvents(Collections.emptySet());
+      if (!BROADCASTERS.isEmpty() || null == subsAlwaysActive || !subsAlwaysActive.isEmpty()
+          // when there are no broadcasters and no known subscriptions, only check for updated events once per minute
+          || (System.currentTimeMillis() - lastUpdateCheck.get()) > TimeUnit.MINUTES.toMillis(1)) {
+
+        lastUpdateCheck.set(System.currentTimeMillis());
+        updateEnabledEvents(Collections.emptySet());
+      }
     }
   }