FileStash's old files cleanup thread bugfixed and cleaned up; improved exception usage in FileStash

The old files cleanup thread now uses new threading.Thread interface, has no hardcoded intervals and is verified to work properly. FileStash now has an internal exception hierarchy and doesn't swallow exceptions silently as often as before.
Implemented cleanup job for tasks

Tasks that have been waiting to get processed for more than a certain time will automatically be cancelled.
Tasks that are older than a certain time will be removed from the list altogether.

Also, renamed the SendorTask.set_queue_info operation to SendorTask.enqueued to better reflect its purpose (as it now logs the timestamp of the enqueued action).