[avalanche] Add invalidateavalancheproof and reconsideravalancheproof RPCs

Summary:
This let a user manually change the status of an avalanche proof. This
is mainly useful in tests to change the vote of a node.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: Klakurka

Differential Revision: https://reviews.bitcoinabc.org/D14744