wallet refactor: Avoid use of Chain::Lock in CWallet::RescanFromTime

Summary:
This is a step toward removing the Chain::Lock class and reducing cs_main
locking.

This change has no effect on behavior.

This is part [8/12] of Core PR17954 : https://github.com/bitcoin/bitcoin/pull/17954/commits/3cb85ac594f115db99f96b0a0f4bfdcd69ef0590

Depends on D7870

Test Plan:

ninja all check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7871