* Add NU5 variant to NetworkUpgrade
* Add consensus branch ID for NU5
* Add network protocol versions for NU5
* Add NU5 to the protocol::version_consistent test
* Make unimplemented panic messages more specific
* Block target spacing doesn't change in NU5
* add comments for future updates for NU5
Co-authored-by: teor <teor@riseup.net>