[backport#15730 2/4] wallet: Track current scanning progress

Summary:
https://github.com/bitcoin/bitcoin/pull/15730/commits/90e27abe37cc84c7b206f20d28aafe32e71e7209

Depends on D7010

Partial backport of Core PR15730

Test Plan:

ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7011