Monitor should drop the connection when faced with an Access Denied error

If monitors don't have the require grants and an operation causes an error to occur, the monitor does not drop the connection to the server. New global grants (e.g. READ_ONLY ADMIN) are only applied to connections created after the grants were given.

Original description:
Customer feature request for MaxScale that allows for the dynamic/real-time reloading/updating for the maxscale database user grants, without requiring a restart of the MaxScale service if it is already running. Currently, new grants only get activated active during maxscale service startup.

For example, in the error message below, the customer granted the missing READ_ONLY ADMIN privilege, but it only became effective after restarting the MaxScale service:

error message displayed was: "[mariadbmon] Setting read_only on 'node-0' failed: 'Access denied; you need (at least one of) the READ_ONLY ADMIN privilege(s) for this operation."