Concrete `rust-bitcoin` type in `hidden` modules

[tcharding]

While reviewing #435 I missed that we added a concrete rust-bitcoin type in the version specific struct GetOrphanTxs. It should have been a list of strings with an into_model function converting to a type from model that has the list of Txids.

In v30 and v29. Found incidentally while I was grepping for Vec<Txid>.