[avalanche] pass the avalanche processor to Chainstate::LoadExternalBlockFile and ThreadImport

Summary:
Another step in removing access to the g_avalanche global in Chainstate code.

Depends on D16266

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16267