Instead of using void there's now a MXS_SPECIFIC_MONITOR struct
from which monitor specific types can be derived. This change
does not bring about other benefits than a bit of clarity but
this is the first step in clearing up the monitor API.
The monitor performs simple monitoring of a group replication cluster. It
expects no extra parameters in addition to the common monitor parameters
and will only set master and slave status bits on the servers. It is a
part of the experimental package as it is a very experimental module.
Further improvements would be to add the usage of the synced state.
