[avalanche] Pass chainman to Processor

Summary: This removes usage of g_chainman and ::ActiveChainstate() in the Avalanche processor.

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: deadalnix, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11702