net_processing: move some globals into PeerManagerImpl

Summary:
nSyncStarted, mapBlockSource, g_outbound_peers_with_protect_from_disconnect were all only used by PeerManagerImpl methods already.

Changes related to g_wtxid_relay_peers are not present because these are for segwit.

This is a backport of core#20942 [1/9]
https://github.com/bitcoin/bitcoin/pull/20942/commits/9781c08a33569370f191b30cc7e2ce9b5317eb3e

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Subscribers: sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D11433