[db] Migration for txindex data to new, separate database.

Summary:
backport of c88bcec93 from core PR13033

Depends D2901

Test Plan: build (no relevant tests in original PR)

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Maniphest Tasks: T617

Differential Revision: https://reviews.bitcoinabc.org/D2903