index: Move index DBs into index/ directory.
backports last two commits from PR13243:
ec3073a27 index: Move index DBs into index/ directory.
89eddcd36 index: Remove TxIndexDB from public interface of TxIndex.
(these diffs are co-dependent and need to be brought in as one)
Test Plan: make check
Reviewers: #bitcoin_abc, deadalnix, Fabien, jasonbcox
Reviewed By: #bitcoin_abc, deadalnix
Maniphest Tasks: T630
Differential Revision: https://reviews.bitcoinabc.org/D2938