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may acquire an out of date mutex.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/mutex_pool_structs.h

@@ -0,0 +1,14 @@
+#ifndef JEMALLOC_INTERNAL_MUTEX_POOL_STRUCTS_H
+#define JEMALLOC_INTERNAL_MUTEX_POOL_STRUCTS_H
+
+/* This file really combines "structs" and "types", but only transitionally. */
+
+/* We do mod reductions by this value, so it should be kept a power of 2. */
+#define MUTEX_POOL_SIZE 256
+
+typedef struct mutex_pool_s mutex_pool_t;
+struct mutex_pool_s {
+	malloc_mutex_t mutexes[MUTEX_POOL_SIZE];
+};
+
+#endif /* JEMALLOC_INTERNAL_MUTEX_POOL_STRUCTS_H */