replication_heartbeat table privilege is not checked / fails silently / is not documented

Hi,

the detect_replication_lag=true directive assumes that the maxscale_schema.replication_heartbeat table exists.

However, if the maxscale user does not have sufficient privileges to create this DB and table, nothing happens (no error message is logged).

IMHO, at least an error should be returned, and the privilege should be checked as well.

Also note that this functionality and privilege isn't currently documented.

Thanks !