move getAvalancheVoteForBlock to PeerManagerImpl

Summary:
It is only called in another PeerManagerImpl method, and it needs access to g_chainman which is about to be replaced by m_chainman throughout net_processing.cpp

Also capitalize the first word, to follow the convention used for all other methods.

Depends on D11495

Test Plan:
With clang and debug
ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D11496