wallet: Add AddWallet, RemoveWallet, GetWallet and GetWallets

Summary:
With these new functions all vpwallets usage are removed
and vpwallets is now a static variable (no external linkage).

This is a partial backport of Core PR13017 : https://github.com/bitcoin/bitcoin/pull/13017/commits/373aee26c3df233f4e0a7e806f45ac7cb5aab1e6

Depends on D3486 and D3487

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3488