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.

test/unit/bin_batching.sh

@@ -0,0 +1,10 @@
+#!/bin/sh
+
+# This value of max_batched_size effectively requires all bins to be batched;
+# our page limits are fuzzy, but we bound slab item counts to 2**32, so we'd be
+# at multi-gigabyte minimum page sizes.
+# The reason for this sort of hacky approach is that we want to
+# allocate/deallocate PAGE/2-sized objects (to trigger the "non-empty" ->
+# "empty" and "non-empty"-> "full" transitions often, which have special
+# handling). But the value of PAGE isn't easily available in test scripts.
+export MALLOC_CONF="narenas:2,bin_shards:1-1000000000:3,max_batched_size:1000000000,remote_free_max_batch:1,remote_free_max:4"