wallet: Make vpwallets usage thread safe

Summary:
This PR turns the functions introduced in #13017 thread safe. This is
required to correctly support dynamically loading wallets, which is
implemented in #10740.

Backport of core PR13028
https://github.com/bitcoin/bitcoin/pull/13028/files

Test Plan:

make check
./test/functional/test_runner.py wallet_*

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3631