[avalanche] Hold the peer manager lock during the peer cleanup

Summary: The cleanup task is accessing and modifying the peer structure, so it needs to hold the cs_peerManager lock.

Test Plan:
With Clang and Debug:

ninja all check-all

With D11735:

./contrib/teamcity/build-configurations.py build-tsan

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D11736