[wallet] Clarify wallet initialization / destruction interface

Summary:
All wallet component initialization/destruction functions are now in their own wallet/init.cpp translation unit and are no longer static functions on the CWallet class. The bitcoin_server also no longer has any knowledge that there are multiple wallets in vpwallet.

There should be no changes in behavior from this PR.

Backport of core PR10767

Test Plan:

make check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2180