Add 'getSubscriptions' API on Server.

Add a test for sessions without clean session flag.

broker/src/test/java/io/moquette/spi/persistence/MemorySessionStore.java

@@ -157,6 +157,15 @@ public class MemorySessionStore implements ISessionsStore {
         return null;
     }
 
+    @Override
+    public List<Subscription> getSubscriptions() {
+        List<Subscription> subscriptions = new ArrayList<>();
+        for (Map.Entry<String, Set<Subscription>> entry : m_persistentSubscriptions.entrySet()) {
+            subscriptions.addAll(entry.getValue());
+        }
+        return subscriptions;
+    }
+
     @Override
     public void inFlightAck(String clientID, int messageID) {
         Map<Integer, MessageGUID> m = this.m_inflightStore.get(clientID);