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.

src/stats.c

@@ -1555,6 +1555,8 @@ stats_general_print(emitter_t *emitter) {
 	OPT_WRITE_BOOL("utrace")
 	OPT_WRITE_BOOL("xmalloc")
 	OPT_WRITE_BOOL("experimental_infallible_new")
+	OPT_WRITE_SIZE_T("max_batched_size")
+	OPT_WRITE_SIZE_T("remote_free_max_batch")
 	OPT_WRITE_BOOL("tcache")
 	OPT_WRITE_SIZE_T("tcache_max")
 	OPT_WRITE_UNSIGNED("tcache_nslots_small_min")