[avalanche] Only select from peers which have an actual node attached
This doesn't change the behavior to the extent that selecting a peer with no node would trigger another selection round, but this wasteful.
Instead, we maintain the node count attached to the peer and act accordingly.
Depends on D8606
Addd code in PeerManager::verify to check that the node count is tracked properly.
Existing tests ensure that the observable behavior is not altered in any way that matters.
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D8610