database: versioning

[JoeGruffins]

Currently objects track their own versioning. We don't have a decided way to update values already in the db to a new version. If the version is incorrect, the only thing to do is to wipe all values for that key and start over. The only way to do that is for the user to delete their wallet manually and restore from seed words. This is a bad ux.

dcrwallet has a seperate updating package and handles all updates at once on start-up. https://github.com/decred/dcrwallet/blob/master/wallet/udb/upgrades.go That seems like a good way to do things to me. If there's a problem it is best to stop at startup rather than half way through doing something when you get a random exception, as will be the case if we have each object handle its own versioning.