use_sql_variables_in=master can break functionality of important session variables

When use_sql_variables_in=master, the ReadWriteSplit router will route queries like the following only to the master:

SET SESSION wsrep_sync_wait=1;

In this specific case, it's very important that wsrep_sync_wait get set on the slaves. If the variable is only routed to the master, the application might see errors when queries don't return the expected results.

https://mariadb.com/kb/en/mariadb/galera-cluster-system-variables/#wsrep_sync_wait

Possible fixes:

• Only let use_sql_variables_in affect user-defined variables, not system variables.

• Only let use_sql_variables_in affect queries that read variables, not queries that write variables with SET.