Make execute_session_command virtual

Making the function allows higher level checking to be done by the derived
class.

The readwritesplit does some of the reply bookkeeping for session commands
in the function. This makes their execution less prone for errors as the
states are always updated correctly whenever a session command is
executed.

include/maxscale/backend.hh

@@ -63,7 +63,7 @@ public:
      *
      * @return True if the command was executed successfully
      */
-    bool execute_session_command();
+    virtual bool execute_session_command();
 
     /**
      * @brief Add a new session command to the tail of the command queue