[avalanche] Implement the challenge/response protocol

Summary:
This implement the voting mechanism, with pools and responses and account for the votes properly. The peer selection is not done at this point in time.

Depends on D2045

Test Plan:

make check

Updated tests and added new ones to reflect the added fonctionalities.

Reviewers: #bitcoin_abc, jasonbcox, schancel, Fabien

Reviewed By: #bitcoin_abc, jasonbcox, schancel, Fabien

Subscribers: Fabien, teamcity, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D2046