[avalanche] Broadcast our local proof once it's been verified

Summary:
This should help propagating our own proof to other peers, because it makes sure at least one of them did download our proof. Privacy is not a problem here because we want other peers to know what our proof is. This is a first step toward better initial broadcast; the next step is to store our valid proofs in a file at shutdown, and broadcast them again upon startup.

Depends on D11278.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, tyler-smith

Reviewed By: #bitcoin_abc, tyler-smith

Differential Revision: https://reviews.bitcoinabc.org/D11280