[avalanche] initialize avalanche before node.peerman

Summary: In a next commit, when g_avalanche is removed, the PeerManager::make() factory will need a pointer to m_node.avalanche. Moving the avalanche initialization before the PeerManager initialization in advance makes the following commit slightly easier to review.

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16223