Fix timestamp inconsistencie

Due to latency, sometime a node can handle the start mining message and send the cross validation message to other node before the other node had handle the start mining message. And so we have a timestamp inconsistencie.
To solve it we add a difference of 3 sec which is the default timeout for the start mining message

lib/archethic/mining/validation_context.ex

@@ -986,7 +986,7 @@ defmodule Archethic.Mining.ValidationContext do
          validation_time: validation_time
        }) do
     diff = DateTime.diff(timestamp, validation_time)
-    diff <= 0 and diff > -10
+    diff <= 3 and diff > -10
   end
 
   defp valid_stamp_signature(stamp = %ValidationStamp{}, %__MODULE__{