Rename and overload adding of session commands

As the session commands are always appended to the end of the list, the
name should reflect that action. For this reason, the function was renamed
to append_session_command.

Readwritesplit supports replacement of slave servers by storing all
executed session commands in a list. To make the copying of this list a
bit cleaner, an overload for a list of session commands was added. This
will allow relatively smooth addition of server replacement to all router
modules that use the Backend class.

server/modules/routing/schemarouter/schemaroutersession.cc

@@ -726,7 +726,7 @@ bool SchemaRouterSession::route_session_write(GWBUF* querybuf, uint8_t command)
         {
             GWBUF *buffer = gwbuf_clone(querybuf);
 
-            (*it)->add_session_command(buffer, m_sent_sescmd);
+            (*it)->append_session_command(buffer, m_sent_sescmd);
 
             if (MXS_LOG_PRIORITY_IS_ENABLED(LOG_INFO))
             {