[avalanche] Add a block registry and facility to register votes on these blocks.

Summary:
This is essentially an interface to use what was created in D2040 in a way that is applicable to the Bitcoin Cash protocol.

Depends on D2040

Test Plan: Added test cases.

Reviewers: #bitcoin_abc, Fabien, schancel

Reviewed By: #bitcoin_abc, Fabien, schancel

Subscribers: teamcity, Fabien, jasonbcox, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2042