[avalanche] Report the list of finalized blocks when votes are accounted for

Summary:
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