walletdb: Add IsBDBWalletLoaded to look for BDB wallets specifically

Summary:
Partial backport (2/5) of core PR19290:
https://github.com/bitcoin/bitcoin/pull/19290/commits/f6fc5f3849bac48dfccd015bec7089cb711d0667

Depends on D8611.

Test Plan:

ninja all check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8612