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/test/testhash.c

@@ -36,8 +36,8 @@
 #include <time.h>
 
 #include <maxscale/alloc.h>
-#include <atomic.h>
-#include <hashtable.h>
+#include <maxscale/atomic.h>
+#include <maxscale/hashtable.h>
 
 static void
 read_lock(HASHTABLE *table)