[net processing] Move net processing consts to net_processing.cpp

Summary:

Following the main.cpp split, there are still some constants in the wrong places, eg net_processing constants in validation.h. Move them all to their rightful homes. At the same time, make them constexpr.

Also move all const declarations to the top of their files, and ensure that they all have doxygen comments.

This is a backport of Core PR17383 [1/4]
https://github.com/bitcoin/bitcoin/pull/17383/commits/b8580cacc70764ba5a48e3defb864d75e6c28626

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8976