[avalanche] Fix a lock order inversion on shutdown

Summary: The lock order is touchy during shutdown and was broken by D9768.

Test Plan:
With debug:

./test/functional/test_runner.py abc_p2p_proof_inventory

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9791