MXS-1740 Hintfilter leaks memory

Single spot where an existing hint ptr was overwritten. Removed gwbuf_add_hint()
because it was  adding hints at the opposite end compared to functions in hint.h.
Added hint_splice() to replace.

include/maxscale/buffer.h

@@ -379,15 +379,6 @@ extern char *gwbuf_get_property(GWBUF *buf, char *name);
  */
 extern GWBUF *gwbuf_make_contiguous(GWBUF *buf);
 
-/**
- * Add hint to a buffer.
- *
- * @param buf   The buffer to add the hint to
- * @param hint  The hint. Note that the ownership of @c hint is transferred
- *              to @c buf.
- */
-extern void gwbuf_add_hint(GWBUF *buf, HINT *hint);
-
 /**
  * Add a buffer object to GWBUF buffer.
  *