epoch: add non-preemptible "critical" variant

adds:

• epoch_enter_critical() - can be called inside a different epoch, starts a section that will acquire any MTX_DEF mutexes or do anything that might sleep.
• epoch_exit_critical() - corresponding exit call
• epoch_wait_critical() - wait variant that is guaranteed that any threads in a section are running.
• epoch_global_critical - an epoch_wait_critical safe epoch instance

Requested by: markj
Approved by: sbruno