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/hashtable.c

@@ -18,8 +18,8 @@
 #include <sys/stat.h>
 #include <fcntl.h>
 #include <maxscale/alloc.h>
-#include <atomic.h>
-#include <hashtable.h>
+#include <maxscale/atomic.h>
+#include <maxscale/hashtable.h>
 
 /**
  * @file hashtable.c General purpose hashtable routines