MXS-1848 Rename monitor types and instance variable

The following type name changes

  MXS_MONITOR_OBJECT   -> MXS_MONITOR_API
  MXS_SPECIFIC_MONITOR -> MXS_MONITOR_INSTANCE

Further, the 'handle' instance variable of what used to be
called MXS_MONITOR_OBJECT has been renamed to 'api'.

An example, what used to look like

   mon->module->stopMonitor(mon->handle);

now looks like

  mon->api->stopMonitor(mon->instance);

which makes it more obvious what is going on.

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 : public MXS_SPECIFIC_MONITOR
+class MariaDBMonitor : public MXS_MONITOR_INSTANCE
 {
 private:
     MariaDBMonitor(const MariaDBMonitor&);