rpc: Optimize serialization disk space of dumptxoutset

Summary:
Co-authored-by: Aurèle Oulès <aurele@oules.com>
Co-authored-by: TheCharlatan <seb.kung@gmail.com>

This is a partial backport of core#29612
https://github.com/bitcoin/bitcoin/pull/29612/commits/de95953d870c41436de67d56c93259bc66fe1434
https://github.com/bitcoin/bitcoin/pull/29612/commits/c14ed7f384075330361df636f40121cf25a066d6
https://github.com/bitcoin/bitcoin/pull/29612/commits/4d8e5edbaa94805be41ae4c8aa2f4bf7aaa276fe

Test Plan:
ninja all check-all

add a assume utxo checkpoint, dump the corresponding utxo set, start a fresh node, load the utxo set and let it fully validate

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17985