https://github.com/bitcoin/bitcoin/pull/13243/files
```
ec3073a27 index: Move index DBs into index/ directory.
89eddcd36 index: Remove TxIndexDB from public interface of TxIndex.
2318affd2 MOVEONLY: Move BaseIndex to its own file.
f376a4924 index: Generalize logged statements in BaseIndex.
61a1226d8 index: Extract logic from TxIndex into reusable base class.
e5af5fc6f db: Make reusable base class for index databases.
9b0ec1a7f db: Remove obsolete methods from CBlockTreeDB.
```