[avalanche] Add the concept of inconclusive votes
Any error code starting with a the bit 1 will be considered as an inconclusive vote. The node is not for or against.
Depends on D2148
Test Plan: Added test cases to make sure this is handled properly.
Reviewers: #bitcoin_abc, jasonbcox
Reviewed By: #bitcoin_abc, jasonbcox
Subscribers: jasonbcox, teamcity, schancel
Differential Revision: https://reviews.bitcoinabc.org/D2160