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/jemalloc.c

@@ -1325,6 +1325,16 @@ malloc_conf_init_helper(sc_data_t *sc_data, unsigned bin_shard_sizes[SC_NBINS],
 				} while (vlen_left > 0);
 				CONF_CONTINUE;
 			}
+			CONF_HANDLE_SIZE_T(opt_bin_info_max_batched_size,
+			    "max_batched_size", 0, SIZE_T_MAX,
+			    CONF_DONT_CHECK_MIN, CONF_DONT_CHECK_MAX,
+			    /* clip */ true)
+			CONF_HANDLE_SIZE_T(opt_bin_info_remote_free_max_batch,
+			    "remote_free_max_batch", 0,
+			    BIN_REMOTE_FREE_ELEMS_MAX,
+			    CONF_DONT_CHECK_MIN, CONF_CHECK_MAX,
+			    /* clip */ true)
+
 			if (CONF_MATCH("tcache_ncached_max")) {
 				bool err = tcache_bin_info_default_init(
 				    v, vlen);