refactor: remove ::vpwallets and related global variables

Summary:
Move global wallet variables to WalletContext struct

This is a backport of core#19101 and core#22992

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17952