db: Make reusable base class for index databases.

Summary:
backported second commit (e5af5fc6f) from PR13243
https://github.com/bitcoin/bitcoin/commit/e5af5fc6f

Depends on D2924

Test Plan: make check

Reviewers: #bitcoin_abc, deadalnix, jasonbcox, Fabien

Reviewed By: #bitcoin_abc, deadalnix

Maniphest Tasks: T630

Differential Revision: https://reviews.bitcoinabc.org/D2934