route the first select after insert on a columnstore table to master

With enabled "insert cache" on columnstore the changes are not visible on the replicas
until the first select was processed on the master.

Therefore Maxscale should forward the first select after an insert to the master node on a
columnstore cluster, even on a read-write router.

"Accumulated inserts are flushed into columnstore on various events (all non inserts, including SELECT - and settable size threshold defaulted to 500,000 rows in the cache)."

https://mariadb.com/docs/columnstore/ref/mdb/system-variables/columnstore_cache_use_import/