Add element acquire/release capabilities to rtree.

This makes it possible to acquire short-term "ownership" of rtree
elements so that it is possible to read an extent pointer *and* read the
extent's contents with a guarantee that the element will not be modified
until the ownership is released.  This is intended as a mechanism for
resolving rtree read/write races rather than as a way to lock extents.

include/jemalloc/internal/chunk.h

@@ -87,7 +87,7 @@ JEMALLOC_INLINE extent_t *
 chunk_lookup(const void *ptr, bool dependent)
 {
 
-	return (rtree_get(&chunks_rtree, (uintptr_t)ptr, dependent));
+	return (rtree_read(&chunks_rtree, (uintptr_t)ptr, dependent));
 }
 #endif