wallet: Avoid use of Chain::Lock in importwallet and dumpwallet

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

This change only affects behavior in the case where wallet last block processed
falls behind the chain tip, in which case it will use more accurate backup and
rescan timestamps.

This is part [5/12] of Core PR17954 : https://github.com/bitcoin/bitcoin/pull/17954/commits/25a9fcf9e53bfa94e8f8b19a4abfda0f444f6b2a

Depends on D7867

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7868