Add batching to arena bins.

This adds a fast-path for threads freeing a small number of allocations to
bins which are not their "home-base" and which encounter lock contention in
attempting to do so. In producer-consumer workflows, such small lock hold times
can cause lock convoying that greatly increases overall bin mutex contention.

src/stats.c

@@ -1556,6 +1556,7 @@ stats_general_print(emitter_t *emitter) {
 	OPT_WRITE_BOOL("xmalloc")
 	OPT_WRITE_BOOL("experimental_infallible_new")
 	OPT_WRITE_SIZE_T("max_batched_size")
+	OPT_WRITE_SIZE_T("remote_free_max")
 	OPT_WRITE_SIZE_T("remote_free_max_batch")
 	OPT_WRITE_BOOL("tcache")
 	OPT_WRITE_SIZE_T("tcache_max")