Keep props small even when using resize

In order to find bad behavior I've been using resize(600,...) to
generate tests.  This make the size of the properties list blow up.
Given that the props don't seem to be a factor in the issues I'm
tracking I'm gonna to constrain this generator for now.
Smarter IFT generator

Instead of using the postings generator and then only using half of
the data write a generator just for IFTs.
Add lock-doesnt-exist counter example

This counter example proves that async lookup/range/iterators can try
to release files that have since been deleted by the `drop` call.
Cleanup async workers on drop

During drop call there could still be async range/lookup/iterator pids
running.  Kill these pids and remove them from the list so that a
release call is not made to a non-existant file.