[avalanche] Move statistics constants into their own header

Summary: This is needed in order to make these constants available to future unit tests that live outside of net_tests. It will come in handy when we introduce a peer availability score that isn't tied down with net code dependencies.

Test Plan:

ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13003