OPTIM: pools: reduce local pool cache size to 512kB

OPTIM: pools: reduce local pool cache size to 512kB

Now that we support batched allocations/releases, it appears that we can
reach the same performance on H2 with shared pools and 256kB thread-local
cache as without shared pools, a fast allocator and 1MB thread-local cache.
With 512kB we're up to 10% faster on highly multiplexed H2 than without the
shared cache. This was tested on a 16-core ARM machine. Thus it's time to
slightly reduce the per-thread memory cost, which may also improve the
performance on machines with smaller L2 caches. It essentially reverts
commit f587003fe ("MINOR: pools: double the local pool cache size to 1 MB").