Sys var and user var combined issue

SET @saved_cs_client= @@character_set_client; // route to master
SET character_set_client = @saved_cs_client; // route to all

So character_set_client will be null at slaves, then session command failed, close slave backends.

propose：
1. QC parse 'SET @saved_cs_client= @@character_set_client;' as QUERY_TYPE_SYSVAR_READ | QUERY_TYPE_USERVAR_WRITE, current is QUERY_TYPE_UNKNOWN

2. route 'QUERY_TYPE_SYSVAR_READ | QUERY_TYPE_USERVAR_WRITE' to all nodes