Fix slave reconnection regression

The state of the backend needs to be checked before any pending session
commands are executed on it.

Added debug assertions to catch invalid use of the status functions of
closed backends.

include/maxscale/backend.hh

@@ -100,6 +100,7 @@ public:
      */
     inline bool has_session_commands() const
     {
+        ss_dassert(in_use());
         return !m_session_commands.empty();
     }