walletdb: Move BDB specific things into bdb.{cpp/h}

Summary:

Leave wallet/db.{cpp/h} for generic WalletDatabase stuff. The BDB
specific stuff goes into bdb.{cpp/h}

Completes backport (5/5) of core PR19290:
https://github.com/bitcoin/bitcoin/pull/19290/commits/61c16339da4e80b1320a6296df6d96cd7a84bb4e

Depends on D8614.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8615