Add metadata for state backups.

This is part of my effort to split up the work for backups into a few smaller patches.
You're probably right.  I figured generating our own ID is easy so there wasn't much incentive to expose that extra info unnecessarily (and there is no guarantee to what info might be encoded into their ID in the future).  Any performance benefit to using mongo's auto ID is insignificant here.  Also, keep in mind that I expect the ID will be in play more often than the full metadata, though ultimately neither should be floating around all that much and even less so insecurely.

I'm willing to switch to mongo's auto ID if you think it will help.  One thing I keep coming back to is the benefit of the ID being more human-consumable.  So something that incorporates the timestamp and a short-but-relatively-unique env ID would be good.  The env component of that would preclude using mongo's auto ID though.  Hmm, you're starting to talk me into generating a better ID...