[wallet] Move getBlockHash from Chain::Lock interface to simple Chain

Summary:
Backport of Core PR16426 part [3/5] : https://github.com/bitcoin/bitcoin/pull/16426/commits/0a76287387950bc9c5b634e95c5cd5fb1029f42d

Depends on D7935

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D7936