index: Extract logic from TxIndex into reusable base class.

Summary:
backported third commit (61a1226d8) from PR13243
https://github.com/bitcoin/bitcoin/commit/61a1226d8

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/D2935