[avalanche] Vote on conflicting txs

Summary:
This lets avalanche accept or reject transactions based on their conflicting state.
This pulls back the conflicting transaction upon rejection, or invalidate it upon finalization of its counterpart.

Depends on D16767.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK, roqqit

Reviewed By: #bitcoin_abc, PiRK, roqqit

Subscribers: PiRK, roqqit

Differential Revision: https://reviews.bitcoinabc.org/D16757