Check batches for upload periodically
Check batches for upload periodically

The uploads were only triggered externally, which means if there is no
external triggers, the uploader will not check if there are unfinished
batches to upload.  This change adds an periodic internal trigger for
uploads.

SiftScience/sift-ios,SiftScience/sift-ios,SiftScience/sift-ios,SiftScience/sift-ios