Protect the rtree/extent interactions with a mutex pool.

Instead of embedding a lock bit in rtree leaf elements, we associate extents
with a small set of mutexes.  This gets us two things:

- We can use the system mutexes.  This (hypothetically) protects us from
  priority inversion, and lets us stop doing a backoff/sleep loop, instead
  opting for precise wakeups from the mutex.
- Cuts down on the number of mutex acquisitions we have to do (from 4 in the
  worst case to two).

We end up simplifying most of the rtree code (which no longer has to deal with
locking or concurrency at all), at the cost of additional complexity in the
extent code: since the mutex protecting the rtree leaf elements is determined by
reading the extent out of those elements, the initial read is racy, so that we
may acquire an out of date mutex.  We re-check the extent in the leaf after
acquiring the mutex to protect us from this race.

include/jemalloc/internal/rtree_structs.h

@@ -2,6 +2,7 @@
 #define JEMALLOC_INTERNAL_RTREE_STRUCTS_H
 
 #include "jemalloc/internal/atomic.h"
+#include "jemalloc/internal/mutex_pool_structs.h"
 
 struct rtree_node_elm_s {
 	atomic_p_t	child; /* (rtree_{node,leaf}_elm_t *) */
@@ -18,13 +19,12 @@ struct rtree_leaf_elm_s {
 	 * x: index
 	 * e: extent
 	 * b: slab
-	 * k: lock
 	 *
-	 *   00000000 xxxxxxxx eeeeeeee [...] eeeeeeee eeee00bk
+	 *   00000000 xxxxxxxx eeeeeeee [...] eeeeeeee eeee000b
 	 */
 	atomic_p_t	le_bits;
 #else
-	atomic_p_t	le_extent; /* (extent_t *), lock in low bit */
+	atomic_p_t	le_extent; /* (extent_t *) */
 	atomic_u_t	le_szind; /* (szind_t) */
 	atomic_b_t	le_slab; /* (bool) */
 #endif