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/ndbclustermon/ndbclustermon.h

@@ -18,7 +18,7 @@
 #include <maxscale/thread.h>
 
 // The handle for an instance of a NDB Cluster Monitor module
-struct NDBC_MONITOR : public MXS_SPECIFIC_MONITOR
+struct NDBC_MONITOR : public MXS_MONITOR_INSTANCE
 {
     THREAD thread;                /**< Monitor thread */
     SPINLOCK lock;                /**< The monitor spinlock */