Turn worker into a C++ class

This is the first step in turning the worker mechanism and everything
around it into a set of C++ classes. In this change, the original C
API is still present, but in subsequent changes that will be removed.

server/core/maxscale/worker.h

@@ -90,7 +90,7 @@ void mxs_worker_shutdown_workers();
  */
 static inline bool mxs_worker_should_shutdown(MXS_WORKER* worker)
 {
-    return worker->should_shutdown;
+    return worker->m_should_shutdown;
 }
 
 MXS_END_DECLS