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may acquire an out of date mutex.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_externs.h

@@ -41,11 +41,5 @@ void rtree_delete(tsdn_t *tsdn, rtree_t *rtree);
 #endif
 rtree_leaf_elm_t *rtree_leaf_elm_lookup_hard(tsdn_t *tsdn, rtree_t *rtree,
     rtree_ctx_t *rtree_ctx, uintptr_t key, bool dependent, bool init_missing);
-void rtree_leaf_elm_witness_acquire(tsdn_t *tsdn, const rtree_t *rtree,
-    uintptr_t key, const rtree_leaf_elm_t *elm);
-void rtree_leaf_elm_witness_access(tsdn_t *tsdn, const rtree_t *rtree,
-    const rtree_leaf_elm_t *elm);
-void rtree_leaf_elm_witness_release(tsdn_t *tsdn, const rtree_t *rtree,
-    const rtree_leaf_elm_t *elm);
 
 #endif /* JEMALLOC_INTERNAL_RTREE_EXTERNS_H */