Implement per arena base allocators.

Add/rename related mallctls:
- Add stats.arenas.<i>.base .
- Rename stats.arenas.<i>.metadata to stats.arenas.<i>.internal .
- Add stats.arenas.<i>.resident .

Modify the arenas.extend mallctl to take an optional (extent_hooks_t *)
argument so that it is possible for all base allocations to be serviced
by the specified extent hooks.

This resolves #463.

include/jemalloc/internal/stats.h

@@ -100,8 +100,9 @@ struct arena_stats_s {
 	uint64_t	nmadvise;
 	uint64_t	purged;
 
-	/* Number of bytes currently allocated for internal metadata. */
-	size_t		metadata; /* Protected via atomic_*_zu(). */
+	size_t		base;
+	size_t		internal; /* Protected via atomic_*_zu(). */
+	size_t		resident;
 
 	size_t		allocated_large;
 	uint64_t	nmalloc_large;