[backport#15730 1/4] wallet: Track scanning duration

Summary:
https://github.com/bitcoin/bitcoin/pull/15730/commits/2ee811e6930cf76ea51e6826fe437ed888688adc

Partial backport of Core PR15730

Test Plan:

ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7010