Refactor rtree to be lock-free.

Recent huge allocation refactoring associates huge allocations with
arenas, but it remains necessary to quickly look up huge allocation
metadata during reallocation/deallocation.  A global radix tree remains
a good solution to this problem, but locking would have become the
primary bottleneck after (upcoming) migration of chunk management from
global to per arena data structures.

This lock-free implementation uses double-checked reads to traverse the
tree, so that in the steady state, each read or write requires only a
single atomic operation.

This implementation also assures that no more than two tree levels
actually exist, through a combination of careful virtual memory
allocation which makes large sparse nodes cheap, and skipping the root
node on x64 (possible because the top 16 bits are all 0 in practice).

include/jemalloc/internal/chunk.h

@@ -35,7 +35,7 @@ extern malloc_mutex_t	chunks_mtx;
 /* Chunk statistics. */
 extern chunk_stats_t	stats_chunks;
 
-extern rtree_t		*chunks_rtree;
+extern rtree_t		chunks_rtree;
 
 extern size_t		chunksize;
 extern size_t		chunksize_mask; /* (chunksize - 1). */