[backport#16728] move-only: move coins statistics utils out of RPC

Summary:
These procedures will later be used in the ChainstateManager to compute
statistics (particularly a content hash) for UTXO sets coming in from
snapshots.

This is a backport of Core PR16728

Test Plan:

ninja check-all

Reviewers: #bitcoin_abc, Fabien, deadalnix

Reviewed By: #bitcoin_abc, Fabien, deadalnix

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6147