Work on TileIdPartitioner.readSplits

Here's what I was thinking. Having an HdfsUtils might be a good start to have a home for stuff like this, though that functionality could go into a companion object for LineScanner, a name I'm not so in love with but don't hate. Feel free to make mods. I think this abstracts out the contention point for this "reading a file line by line but maybe it's not there, and we might have to close it different ways" code bit, and makes the readSplits code clean of it. Let me know what you think.