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.

server/core/thread.c

@@ -10,7 +10,7 @@
  * of this software will be governed by version 2 or later of the General
  * Public License.
  */
-#include <thread.h>
+#include <maxscale/thread.h>
 
 /**
  * @file thread.c  - Implementation of thread related operations