[avalanche] don't use global g_avalanche in net_processing

Summary:
This diff is split out of D16224.

Pass the avalanche processor as a const pointer in the PeerManagerImpl constructor.
In most unit tests we currently pass it nullptr, except in processor_tests. This preserves the current behavior.

Depends on D16255

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16256