Revert or Support fully regtest

[jimmysong]

Currently, it's supported in a few places, but not all of them. It should either be fully supported or fully unsupported.

[wip-abramson]

I am currently trying to work with this library and regtest, so would like to see full support if possible. I may be able to help with a PR around this

[wip-abramson]

I think part of the challenge might be implementing the TxFetcher such that it queries a regtest network over an exposed RPC endpoint.