hdf5

The first implementation seemed to allow for the possibility that a thread
could block at the barrier, wake and exit the barrier, re-acquire the barrier
lock and increase `nentered` before the other blocked threads woke and checked
`nentered % count == 0`.  Then the other blocked threads would check `nentered
% count == 0` and, finding it false, go back to sleep in the barrier.  This new
implementation waits for a looser condition to obtain so that threads don't go
back to sleep in the barrier.