net_processing: move PeerManagerImpl into cpp file

Summary:
Partial backport of core#20811:
https://github.com/bitcoin/bitcoin/pull/20811/commits/e0f2e6d2df7117a8dbf17c63c5149fc53a6fe2b2

This is a move only change. The double namespace is kept to limit later merge conflicts, it's harmless.

Depends on D10887.

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/D10890