wallet: Avoid use of Chain::Lock in tryGetTxStatus and tryGetBalances

Summary:
This is a step toward removing the Chain::Lock class and reducing cs_main
locking.

It also helps ensure the GUI display stays up to date in the case where the
node chain height runs ahead of wallet last block processed height.

This is part [2/12] of Core PR17954 : https://github.com/bitcoin/bitcoin/pull/17954/commits/f6da44ccce4cfff53433e665305a6fe0a01364e4

Depends on D7864

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7865