[backport#17304 17/18] Refactor: Move GetKeypoolSize code out of CWallet

Summary:
This commit does not change behavior.

https://github.com/bitcoin/bitcoin/pull/17304/commits/7ef47b88e67718766c92d23973742d08436176e0

Depends on D7163

Partial backport of Core PR17304

Test Plan:

ninja check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7164