Big refactory on session management:

- introduced a unique point of access and management of sessions state (SessionRepository)
- separated behaiour for sessions with clean flag to false into DurableSession
- separated behaviour for session with clean flag to true into TransientSession

broker/src/main/java/io/moquette/server/ConnectionDescriptor.java

@@ -126,7 +126,7 @@ public class ConnectionDescriptor {
 
     @Override
     public String toString() {
-        return "ConnectionDescriptor{" + "clientID=" + clientID + ", cleanSession=" + cleanSession + ", state="
+        return "ConnectionDescriptor{" + "clientID=" + clientID + ", removeTemporaryQoS2=" + cleanSession + ", state="
                 + channelState.get() + '}';
     }