net processing: Move peer_map to PeerManager

Summary:

This moves g_peer_map from a global in net_processing.cpp's unnamed namespace to being a member m_peer_map of PeerManager.

Backport of core#19910.

Ref T1696.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1696

Differential Revision: https://reviews.bitcoinabc.org/D10862