[avalanche] Don't keep stalled transactions around

Summary: With preconsensus if a tx is stalled, there is nothing we can do with it, it won't get mined. This this case we just drop it temporarly but allow for downloading again so we can restart a new poll round later.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18519