[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