alter pool iterator grinder to work in its new location
tweak some test parameters to test more stressfully
enhance iterator test to be more like pool grinder
update with a more minimal fix and a better understanding of the
problem.

For the record:
  Our deadlock occurred when the __all_claimed check passed, but when
another thread released the last element in the pool before we took
back up the lock.  Doing the check inside of the lock (thus preventing
mutation of the claimed state for elements) closes the loophole for the
race.
  Making the pool lock re-entrant and setting the condition variable
might not be required, but it affords us a little bit of extra safety.
pep8 fixes, decrease iterator test time