[avalanche] pass the processor to Chainstate::ProcessNewBlock

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

This adds some g_avalanche callsites in RPC commands, but they will be removed in a later diff when the node context owns avalanche.
Depends on D16267

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16268