MXS-1848 Introduce a specific monitor type

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.

server/modules/monitor/mariadbmon/mariadbmon.hh

@@ -35,7 +35,7 @@ typedef std::tr1::unordered_map<MXS_MONITORED_SERVER*, MariaDBServer*> ServerInf
 typedef std::vector<MariaDBServer*> ServerArray;
 
 // MariaDB Monitor instance data
-class MariaDBMonitor
+class MariaDBMonitor : public MXS_SPECIFIC_MONITOR
 {
 private:
     MariaDBMonitor(const MariaDBMonitor&);