MXS-2271 Move some monitor management functions to an internal class

The class MonitorManager contains monitor-related functions that should not
be called from modules. MonitorManager can access private fields and methods
of the monitor.

server/core/test/test_modulecmd.cc

@@ -407,7 +407,7 @@ int test_domain_matching(const char* actual_module,
     /** Create a monitor */
     char* libdir = MXS_STRDUP_A("../../modules/monitor/mariadbmon/");
     set_libdir(libdir);
-    monitor_create(name, actual_module, NULL);
+    MonitorManager::create_monitor(name, actual_module, NULL);
 
     const MODULECMD* cmd;