txorphanage: Extract GetOrphanTx

Summary:
Extract orphan lookup code into GetOrphanTx function.

This is a backport of core#21148 [6/14]
https://github.com/bitcoin/bitcoin/pull/21148/commits/f294da727413210fda279afdc206a4dd12046d56

Depends on D11485

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11486