Use ph instead of rb tree for extents_avail_

There does not seem to be any overlap between usage of
extent_avail and extent_heap, so we can use the same hook.

The only remaining usage of rb trees is in the profiling code,
which has some 'interesting' iteration constraints.

Fixes #888

src/extent.c

@@ -117,7 +117,7 @@ static void extent_record(tsdn_t *tsdn, arena_t *arena,
 
 /******************************************************************************/
 
-rb_gen(UNUSED, extent_avail_, extent_tree_t, extent_t, rb_link,
+ph_gen(UNUSED, extent_avail_, extent_tree_t, extent_t, ph_link,
     extent_esnead_comp)
 
 typedef enum {