Move headers from server/include to include/maxscale

- Headers now to be included as <maxscale/xyz.h>
- First step, no cleanup of headers has been made. Only moving
  from one place to another + necessary modifications.

include/maxscale/hk_heartbeat.h

@@ -0,0 +1,24 @@
+#ifndef _HK_HEARTBEAT_H
+#define _HK_HEARTBEAT_H
+
+/*
+ * Copyright (c) 2016 MariaDB Corporation Ab
+ *
+ * Use of this software is governed by the Business Source License included
+ * in the LICENSE.TXT file and at www.mariadb.com/bsl.
+ *
+ * Change Date: 2019-07-01
+ *
+ * On the date above, in accordance with the Business Source License, use
+ * of this software will be governed by version 2 or later of the General
+ * Public License.
+ */
+
+/**
+ * The global housekeeper heartbeat value. This value is incremented
+ * every 100 milliseconds and may be used for crude timing etc.
+ */
+
+extern long hkheartbeat;
+
+#endif