Fixed ExpirationService not shutting down Executor (#835)

When shutting down the server the Executor Threads in ScheduledExpirationService instances were not terminated, resulting in a hanging server that would not shut down.

broker/src/main/java/io/moquette/broker/PostOffice.java

@@ -1019,6 +1019,7 @@ public RouteResult routeCommand(String clientId, String actionDescription, Calla
 
     public void terminate() {
         willExpirationService.shutdown();
+        retainedMessagesExpirationService.shutdown();
         sessionLoops.terminate();
     }
 

broker/src/main/java/io/moquette/broker/scheduler/ScheduledExpirationService.java

@@ -25,13 +25,14 @@ public class ScheduledExpirationService<T extends Expirable> {
     private final ScheduledFuture<?> expiredEntityTask;
     private final Clock clock;
     private final Consumer<T> action;
+    private final ScheduledExecutorService actionsExecutor;
 
     private final Map<String, ExpirableTracker<T>> expiringEntitiesCache = new HashMap<>();
 
     public ScheduledExpirationService(Clock clock, Consumer<T> action) {
         this.clock = clock;
         this.action = action;
-        ScheduledExecutorService actionsExecutor = Executors.newSingleThreadScheduledExecutor();
+        this.actionsExecutor = Executors.newSingleThreadScheduledExecutor();
         this.expiredEntityTask = actionsExecutor.scheduleWithFixedDelay(this::checkExpiredEntities,
             FIRER_TASK_INTERVAL.getSeconds(), FIRER_TASK_INTERVAL.getSeconds(),
             TimeUnit.SECONDS);
@@ -71,5 +72,6 @@ public void shutdown() {
             LOG.warn("Can't cancel the execution of expired entities task, was already cancelled? {}, was done? {}",
                 expiredEntityTask.isCancelled(), expiredEntityTask.isDone());
         }
+        actionsExecutor.shutdownNow();
     }
 }