In my benchmarks, using this pool allocator for CCoinsMap gives about
20% faster `-reindex-chainstate` with -dbcache=5000 with practically the
same memory usage. The change in max RSS changed was 0.3%.
The `validation_flush_tests` tests need to be updated because
memory allocation is now done in large pools instead of one node at a
time, so the limits need to be updated accordingly.
This is a partial backport of core#25325
https://github.com/bitcoin/bitcoin/pull/25325/commits/9f947fc3d4b779f017332135323b34e8f216f613
with a fix from core#28913
Depends on D16177