Fix potential keyfolders count leak

As @engelsanchez pointed out in #104  and I forgot/ignored, it's possible for a killed or crashed folder to not release its resource, even when its handle is garbage collected.  This could make a pending fold never get merged, which is really, really bad.

This fix makes sure that all iterating handles do their release properly, no matter by which path they're collected.

This isn't critical for beta, but it really needs to go in before the 2.0 release.