Tcache batching: Plumbing

In the next commit, we'll start using the batcher to eliminate mutex traffic.
To avoid cluttering up that commit with the random bits of busy-work it entails,
we'll centralize them here.  This commit introduces:
- A batched bin type.
- The ability to mix batched and unbatched bins in the arena.
- Conf parsing to set batches per size and a max batched size.
- mallctl access to the corresponding opt-namespace keys.
- Stats output of the above.

test/analyze/sizes.c

@@ -35,6 +35,8 @@ main(void) {
 	P(arena_t);
 	P(arena_stats_t);
 	P(base_t);
+	P(bin_t);
+	P(bin_with_batch_t);
 	P(decay_t);
 	P(edata_t);
 	P(ecache_t);