[backport#15901 1/2] wallet: log on rescan completion

Summary:
https://github.com/bitcoin/bitcoin/pull/15901/commits/6ad372a973cfa3bd0988b2e40dff0a004db1cf8d

Partial backport of Core PR15901

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7032