go/zodb: Fix TxnStatus constants type

Previously TxnComplete was typed, but TxnPacked and TxnInprogress were
untyped constants. Make all constants have TxnStatus type.

go/zodb/zodb.go

@@ -151,8 +151,8 @@ type TxnStatus byte
 
 const (
 	TxnComplete   TxnStatus = ' ' // completed transaction that hasn't been packed
-	TxnPacked             = 'p' // completed transaction that has been packed
-	TxnInprogress         = 'c' // checkpoint -- a transaction in progress; it's been thru vote() but not finish()
+	TxnPacked     TxnStatus = 'p' // completed transaction that has been packed
+	TxnInprogress TxnStatus = 'c' // checkpoint -- a transaction in progress; it's been thru vote() but not finish()
 )