wallet: Avoid use of Chain::Lock in CWallet::CreateTransaction

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, where it may set a different lock time.

This is part [11/12] of Core PR17954 : https://github.com/bitcoin/bitcoin/pull/17954/commits/e958ff9ab5607da2cd321f29fc785a6d359e44f4

Depends on D7873

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7874