Switch to per-UM setting for replication slave

ColumnStore needs to set whether an individual UM should replay slave events on a ColumnStore table.

This will be done with a new MariaDB variable called "columnstore_replication_slave" which is off by default and can only be changed in my.cnf, not at runtime.

It cannot be changed at runtime because it would be a global variable that would need to be re-read, which requires the replication thread to end and restart. This overly complicates things and therefore a MariaDB restart is easier.