walletdb: don't read db every time that a new WalletBatch is created, fix last client version update

Summary:
Better to perform the action only one time (during 'LoadWallet').
Where the value is being used.

The last client value was only being updated launching releases with higher version numbers
and not if the user launched a previous release.

Co-authored-by: MacroFake <falke.marco@gmail.com>

This is a backport of core#25383

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17951