rpc: various fixups for dumptxoutset
Summary:
- Actually generate an assumeutxo hash and display it
- Add nchaintx to output (necessary for use in chainparams entry)
The source PR actually duplicated the "path" field in the output. Don't do that.
This is a backport of core#23155
Depends on D14657
Test Plan: ninja all check-all
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Subscribers: Fabien
Differential Revision: https://reviews.bitcoinabc.org/D14669