Allow weakref callback to be a no-op during iterator cleanup

[wbolster]

When running the tests on Pypy, sometimes this error occurs:

Exception ignored in: weakref callback:
<cyfunction __init__.<locals>.<lambda> at 0x227aec0>

  Traceback (most recent call last):
    File "plyvel/_plyvel.pyx", line 695, in plyvel._plyvel.BaseIterator.__init__.lambda
      lambda wr: ref_dict.pop(iterator_id))
  KeyError: 140185035039968

This issue manifests only sporadically, and may be related to cleanup
order (garbage collection etc.) being different on CPython and Pypy.

LevelDB requires that iterators are closed before a database is closed,
so plyvel tracks ‘active iterators’ using weak references (‘ref_dict’ in
the traceback above). Active iterators are cleaned up when a database is
closed. Apparently the weakref callback sometimes triggers after the
weakref has already been removed from the collection, and this raises
the KeyError. Ignoring this should be fine since it means the iterator
is closed already.

Simplify a bit by using a set instead of a dict to track the weakrefs,
and use set.discard() in the callback, which is a no-op (no exception)
for missing items.

See #140.

[wbolster]

@sbraz any chance you can give this branch a spin?

i tried many times and don't see any errors anymore. the other error in #140 is very puzzling, and perhaps it's also fixed by this PR, but i'm not sure