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/internal/maxscale.hh

@@ -18,8 +18,6 @@
 
 #include <maxscale/maxscale.h>
 
-MXS_BEGIN_DECLS
-
 /**
  * Initiate shutdown of MaxScale.
  *
@@ -39,4 +37,6 @@ void maxscale_reset_starttime(void);
 bool maxscale_teardown_in_progress();
 void maxscale_start_teardown();
 
-MXS_END_DECLS
+bool running_in_admin_thread();
+
+void set_admin_worker(const mxb::Worker* admin_worker);