wallet: Avoid use of Chain::Lock in importmulti

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 may use a more accurate rescan
time.

This is part [6/12] of Core PR17954 : https://github.com/bitcoin/bitcoin/pull/17954/commits/bc96a9bfc61afdb696fb92cb644ed5fc3d1793f1

Depends on D7868

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7869