[net] Tighten scope in net_processing

Summary:
This is a backport of Core PR13417:

• Rescope g_enable_bip61 to net_processing
• [move-only] Move things only referenced in net_processing out of header file
• Restrict as much as possible in net_processing to translation unit

Mark everything else static or in an anonymous namespace.

• Make g_enable_bip61 a member variable of PeerLogicValidation

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3965