Add stop_block out arg to ScanForWalletTransactions

Summary:

Accurately reports the last block successfully scanned, replacing a
return of the chain tip, which represented possibly inaccurated data in
a race condition.

Completes backport of core PR13076:
https://github.com/bitcoin/bitcoin/pull/13076/commits/bd3b0361d840bff95988a048abf70ade94d80524

Depends on D5562.

Test Plan:

ninja all check check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5563