This changes the PoolAllocator to default the alignment to the given type. This makes the code simpler, and most importantly
fixes a bug on ARM 32bit that caused OOM: The class CTxOut has a member CAmount which is an int64_t and on ARM 32bit int64_t
are 8 byte aligned which is larger than the pointer alignment of 4 bytes. So for CCoinsMap to be able to use the pool, we
need to use the alignment of the member instead of just alignof(void*).
A memory resource similar to std::pmr::unsynchronized_pool_resource, but
optimized for node-based containers.
Co-Authored-By: Pieter Wuille <pieter@wuille.net>
