[avalanche] Ignore duplicated avahello messages

Summary: And more generally don't allow a node to override its delegation without restarting. This is not a big deal because there is no such facility (yet) in the node, and it prevents a peer from sending messages repeatidly and use the node CPU for no reason.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D11364