[avalanche] Record peers that have avalanche enabled

Summary:
m_avalanche_state is currently serving a dual purpose: to store state of peers that have been polled
and to determine if a peer supports avalanche. This patch splits those responsibilities apart so that
the latter is recorded as a new state m_avalanche_enabled. This will become more relevant as additional
refactors are done to fix a TSAN data race related to m_avalanche_state.

There is no change in behavior.

This patch also includes some refactors and redundant checks to make the code easier to understand and
refactor further.

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12325