mempool: fix memory loading error on 32-bit machines (#3969)
* Correct memory alignment for 32-bit machine.
Switching the `txsBytes` and `rechecking` fields of `CListMempool` to ensure the correct memory alignment for `atomic.LoadInt64` on 32-bit machine.
* Update CHANGELOG_PENDING.md for #3968Fixed#3968 `mempool` Memory Loading Error on 32-bit Ubuntu 16.04 Machine.
* Update CHANGELOG_PENDING.md
Co-Authored-By: Anton Kaliaev <anton.kalyaev@gmail.com>