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.cc

@@ -912,7 +912,8 @@ bool MariaDBMonitor::check_sql_files()
  * @param params Configuration parameters
  * @return A pointer to MariaDBMonitor specific data. Should be stored in MXS_MONITOR's "handle"-field.
  */
-static void* startMonitor(MXS_MONITOR *monitor, const MXS_CONFIG_PARAMETER* params)
+static MXS_SPECIFIC_MONITOR* startMonitor(MXS_MONITOR *monitor,
+                                          const MXS_CONFIG_PARAMETER* params)
 {
     return MariaDBMonitor::start(monitor, params);
 }