Move {Load,Dump}Mempool to kernel namespace

Summary:
This is mostly moving code. Can be checked with git show --color-moved=dimmed-zebra --color-moved-ws=ignore-all-space

This concludes backport of core#25487
https://github.com/bitcoin/bitcoin/pull/25487/commits/cb3e9a1e3f8d72daaa361fc45dd853775e754b9d

Depends on D15723

Test Plan:

cmake .. -GNinja -DBUILD_BITCOIN_CHAINSTATE=ON

ninja all check-all bitcoin-fuzzers

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15724