Improve main thread detection

Now properly checks if configuration and diagnostics functions are ran in either
main() or in the admin worker. This is useful for debugging and enforcing
thread safety.

Also, monitors are now started and stopped in the admin worker.

server/core/monitor.cc

@@ -46,6 +46,7 @@
 
 #include "internal/config.hh"
 #include "internal/externcmd.hh"
+#include "internal/maxscale.hh"
 #include "internal/monitor.hh"
 #include "internal/modules.hh"
 #include "internal/server.hh"
@@ -1212,8 +1213,7 @@ string Monitor::get_server_monitor(const SERVER* server)
 
 bool Monitor::is_admin_thread()
 {
-    mxb::Worker* current = mxb::Worker::get_current();
-    return current == nullptr || current == mxs_rworker_get(MXS_RWORKER_MAIN);
+    return running_in_admin_thread();
 }
 
 /**