workers hang without logging anything

[tmills]

Sometimes a worker will stop and hang without quitting or notifying anybody. It doesn't return a parse so the iteration never ends and all the other workers end up timing out.

[tmills]

First pass fix is to have the sentence server timeout on workers first. This is implemented so the system will be usable but it is not a long term fix so I will not close the issue yet. Needs worker-level debugging.