[avalanche] Limit the number of delegation levels

Summary: There is no known real world use case for more than a couple delegation levels, and this will prevent a peer from sending avahello message maxing out the levels, which will take some time to verify for no reason.

Test Plan:

ninja check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D11393