✘64-bit time_t on glibc 2.34 and up

[Hal Murray]

If we make any changes to SHM, we should switch to a setup where the memory is 
read only.  The idea is to allow multiple readers.

The trick to implementing that is to have 2 counters.
  X and Y are initialized to the same value.
  The writer bumps X, updates the data, then bumps Y.
  The reader grabs Y, grabs the data, then grabs X.
    If X and Y are the same the data is valid.  If not, try again.



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