[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.
With Clang and Debug:
ninja all check-all
Reviewers: #bitcoin_abc, sdulfari
Reviewed By: #bitcoin_abc, sdulfari
Differential Revision: https://reviews.bitcoinabc.org/D11736