This RPC allows the client to retrieve the file system locations
of the confirmed blocks and their undo data, to allow building
efficient indexes outside of Bitcoin Core.
An example usage is described here:
https://github.com/romanz/electrs/issues/308
By using the new RPC, it is possible to build an address-based
index taking ~24GB and a txindex taking ~6GB (as of Dec. 2020).
