MOVEONLY: Move BaseIndex to its own file.

Summary:
backported fifth commit (2318affd2) from PR13243
https://github.com/bitcoin/bitcoin/commit/2318affd2

Depends on D2936

Test Plan: make check

Reviewers: #bitcoin_abc, deadalnix, Fabien, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix

Maniphest Tasks: T630

Differential Revision: https://reviews.bitcoinabc.org/D2937