[net processing] Add Orphanage empty consistency check
Summary:
When removing the final peer, assert that m_tx_orphanage is empty.
This is a partial backport of core#21562
https://github.com/bitcoin/bitcoin/pull/21562/commits/9190b01d8dcf03b74e9b9e1653688a97ac171b37
Test Plan: ninja all check-all
Reviewers: #bitcoin_abc, sdulfari
Reviewed By: #bitcoin_abc, sdulfari
Differential Revision: https://reviews.bitcoinabc.org/D13377