Use Metadata instead of a third file.

At JoeC's suggestion, use SequenceFile.Metadata like we used to use
HFile's fileInfo block. There's a little hackery required to get access
to the metadata, but it's still better than a separate info.tsv, and
hopefully we can make a change upstream to expose the info more nicely.
code review comments from blackmad

some tests for MMapInputStream. It could definitely use more, but better
than nothing, and would've caught some of the bugs I've written as I
went along.