add txn uncertainty test & fix a bug

Added missing Txn Uncertainty test and along the way realized that forgetting to add ReadWithinUncertaintyIntervalError to GoError() is not a great thing to do as the error simply evaporates.
The old util.CachedRand was initialized using crypt/urandom and other than that is just a plain old rand.Rand, except for not being safe for concurrent access. I noticed this when my `make testrace` failed and removed util.CachedRand from MakePriority (which is the only place it is used outside of MultiRaft, where the context is non-concurrent).
Since we wouldn't want TxnIds to be the same for each server for each run, I'm seeding it here.

Unfortunately math/rand doesn't supply a LockedSource type, even though they have it internally to expose the helper functions we're using for this now.