Allow InterceptHandler to specify the messages they can intercept. This allows us to improve the performance of the broker interceptor, and will make log traces more clear.

- Add methods to return an unique identifier and the supported message types at the InterceptHandler interface.
- Include a default implementation of the new methods in the AbstractInterceptHandler. By default, all the interceptors will be able to handle any message (same as before).
- Comment verifyNoMoreInteractions() call from the BrokerInterceptorTest.testAddAndRemoveInterceptHandler(). The removeInterceptHandler() method of the broker interceptor
  polls the InterceptHandler, so an interaction will always exist after the call to onSubscribe().

broker/src/main/java/io/moquette/interception/messages/InterceptMessage.java

@@ -0,0 +1,10 @@
+package io.moquette.interception.messages;
+
+/**
+ * An interface that sets the root of the interceptor messages type hierarchy.
+ * @author lbarrios
+ *
+ */
+public interface InterceptMessage {
+
+}