K 10
svn:author
V 5
mmacy
K 8
svn:date
V 27
2018-05-18T01:52:51.344969Z
K 7
svn:log
V 477
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

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