[Wallet] refactor CWallet/CWalletDB/CDB

Summary:
Try to hide CDB/bitdb behinde CWalletDB.
Prepare for full wallet database abstraction.

Backport of core's PR8574

Depends on D908

Test Plan:

make check

Reviewers: schancel, #bitcoin_abc

Reviewed By: schancel, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D910