New SHM

[Hal Murray]

Your current code has 2 1/2 memory barriers.  That's the same as my 2 counter 
proposal.

As long as we are mucking in this area, should we take the opportunity and do 
a big jump and convert to a new way of doing things?
  Support old and new SHM until we can drop old.
  Use 2 counters, read only for readers.
  Include a version #
  Avoid time_t in the layout.  Use uint64_t for seconds.
  What else?


-- 
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