Use mockable time everywhere in net_processing
Summary:
The fact that net_processing uses a mix of mockable tand non-mockable time functions made it hard to write functional tests for #19988. I'm opening this as a separate PR as I believe it's independently useful. In some ways this doesn't go quite as far as it could, as there are now several data structures that could be converted to std::chrono types as well now. I haven't done that here, but I'm happy to reconsider that.
Backport of core#20027.
Test Plan:
ninja all check-all
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D9542