Deduplicate missing parents of orphan transactions

Summary:
In the logic for requesting missing parents of orphan transactions, parent
transactions with multiple outputs being spent by the given orphan were being
processed multiple times. Fix this by deduplicating the set of missing parent
txids first.

Co-authored-by: Anthony Towns <aj@erisian.com.au>

This is a backport of core#19596 [2/2]
https://github.com/bitcoin/bitcoin/pull/19596/commits/4c0731f9c50b0556f8a57b912c8f295c7a9ea89c

Depends on D10066

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D10067