[avalanche] Log the parent txids of a finalized child txs

Summary: When a tx is finalized, the event is logged. But this may be a child tx that finalizes its parents implicitely, so make sure to log these as well.

Test Plan:

./test/functional/test_runner.py abc_p2p_avalanche_transaction_finalization

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18488