[avalanche] Report the list of finalized blocks when votes are accounted for
AvalancheProcessor doesn't need to keep track of finalized items. Instead, it can forget about them let the caller know about finalization.
Depends on D2121
Test Plan: Modifed unit tests to reflect the change.
Reviewers: #bitcoin_abc, jasonbcox, schancel, Fabien
Reviewed By: #bitcoin_abc, jasonbcox, schancel, Fabien
Subscribers: Fabien, jasonbcox, teamcity
Differential Revision: https://reviews.bitcoinabc.org/D2045