(#191) - parse the indexable string

I'm testing to see if we can get any performance benefits from parsing the indexable string (in order to get the doc id and key) rather than explicitly writing out the docid and key.  Even if we don't get a performance boost, it'll take up less space on disk, which is a win.
It's okay for istanbul to ignore this, because this code is already covered by a migration test in pouchdb.