Fixed checkStyle and ErrorProne warns

broker/src/main/java/io/moquette/persistence/MemoryMessagesStore.java

@@ -41,7 +41,7 @@ public class MemoryMessagesStore implements IMessagesStore {
     public void storeRetained(Topic topic, StoredMessage storedMessage) {
         LOG.debug("Store retained message for topic={}, CId={}", topic, storedMessage.getClientID());
         if (storedMessage.getClientID() == null) {
-            throw new IllegalArgumentException( "Message to be persisted must have a not null client ID");
+            throw new IllegalArgumentException("Message to be persisted must have a not null client ID");
         }
         m_retainedStore.put(topic, storedMessage);
     }

broker/src/main/java/io/moquette/persistence/MemorySessionStore.java

@@ -33,7 +33,7 @@ public class MemorySessionStore implements ISessionsStore, ISubscriptionsStore {
 
     private static final Logger LOG = LoggerFactory.getLogger(MemorySessionStore.class);
 
-    class Session {
+    static class Session {
         final String clientID;
         final Map<Topic, Subscription> subscriptions = new ConcurrentHashMap<>();
         boolean cleanSession;
@@ -170,7 +170,6 @@ public class MemorySessionStore implements ISessionsStore, ISubscriptionsStore {
         return subscriptions.get(subcription.getTopicFilter());
     }
 
-
     @Override
     public StoredMessage inFlightAck(String clientID, int messageID) {
         return getSession(clientID).outboundFlightMessages.remove(messageID);