Clean up iterators from disk

[msm-code]

Feature Category

• [ ] Correctness
• [ ] User Interface / User Experience
• [ ] Performance
• [x] Other (please explain)

Describe the problem

This is just a nitpick (after iterator GC was added to ursadb), but mquery could at least try to avoid leaving too many iterators on disk.

Describe the solution you'd like

Especially "problematic" are:

• empty iterators (if query returns 0 files, iterator is still created but never read from so never deleted)
• cancelled queries (iterators won't be clened up)

Describe alternatives you've considered

Just do nothing, they will be deleted after 1 day (by default).