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

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 will treat the last block processed as the
current tip.

This is part [12/12] of Core PR17954 : https://github.com/bitcoin/bitcoin/pull/17954/commits/48973402d8bccb673eaeb68b7aa86faa39d3cb8a

Depends on D7874

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7875