MXS-1503: Remove redundant code

Moved session command execution into the Backend class itself as the
session commands are defined as a related part of it. This allows all
connections to execute session commands if some are available.

Removed explicit SERVER_REF usage in the readwritesplit connection
creation code and replaced it with SRWBackend. This allows the removal of
the get_root_master_backend function which duplicated the functionality in
get_root_master.

include/maxscale/backend.hh

@@ -143,10 +143,11 @@ public:
      * @brief Create a new connection
      *
      * @param session The session to which the connection is linked
+     * @param sescmd  Pointer to a list of session commands to execute
      *
      * @return True if connection was successfully created
      */
-    bool connect(MXS_SESSION* session);
+    bool connect(MXS_SESSION* session, SessionCommandList* sescmd = NULL);
 
     /**
      * @brief Close the backend