[avalanche] Fix inconsistent numeric types

Summary: Method arguments are aligned type-wise with their member type counterparts.

Test Plan: did rebuild and rerun unit tests

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2174