- Move global network epoch definition to epoch.h, as more different

- Move global network epoch definition to epoch.h, as more different
  subsystems tend to need to know about it, and including if_var.h is
  huge header pollution for them.  Polluting possible non-network
  users with single symbol seems much lesser evil.
- Remove non-preemptible network epoch.  Not used yet, and unlikely
  to get used in close future.