[avalanche] pass the processor to Chainstate::AvalancheFinalizeBlock

Summary:
Pass a reference rather than a pointer, so that this function only ever gets called if there is an avalanche processor initialized.

This concludes the removal of g_avalanche access in Chainstate functions.

Depends on D16313

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16270