MXS-852: Store the internal ID in the buffer

If the internal ID is stored in the buffer when it is moving inside the
readwritesplit router, the RWBackend can manage the execution of all
commands with a statement ID by replacing the stored ID with the correct
value.

server/core/backend.cc

@@ -86,7 +86,7 @@ bool Backend::execute_session_command()
 
     SessionCommandList::iterator iter = m_session_commands.begin();
     SessionCommand& sescmd = *(*iter);
-    GWBUF *buffer = sescmd.copy_buffer().release();
+    GWBUF *buffer = sescmd.deep_copy_buffer();
     bool rval = false;
 
     switch (sescmd.get_command())