wallet: Avoid use of Chain::Lock in rescanblockchain

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. The rescanblockchain error height error checking
will just be stricter in this case and only accept values up to the last
processed height

This is part [9/12] of Core PR17954 : https://github.com/bitcoin/bitcoin/pull/17954/commits/1be8ff280c78c30baabae9429c53c0bebb89c44d

Depends on D7871

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7872