Simplify the way we handle validations. Include a signing time instead of a

closing time. Keep only the validation with the most recent signing time.
Sign using network time. This eliminates the ValidationPair nightmare and
makes the logic must easier to understand, increasing confidence that it
does what it's supposed to do.

src/SerializedObject.h

@@ -92,6 +92,7 @@ enum SOE_Field
 	sfSequence,
 	sfSignature,
 	sfSigningKey,
+	sfSigningTime,
 	sfSourceTag,
 	sfTakerGets,
 	sfTakerPays,