Drop messages that are no longer useful for GPBFT progression

As a GPBFT instance progresses some messages become irrelevant, in that
they do not effectively aid the progress of the instance for
participants. Instead, GPBFT offers other built-in mechanisms to aid
progress of lagging participants such as selective
rebroadcast, propagation of DECIDE messages from the previous instance
and finality certificate exchange.

The changes here introduce a dedicated error type returned as part of
message validation to signal that although a message is valid it is no
longer relevant. This error type is then checked by pubsub to avoid
further propagation of those messages.

This reduces the redundant pubsub traffic for the network participants.

Fixes #583

gpbft/errors.go

@@ -25,6 +25,9 @@ var (
 	//
 	// See SupplementalData.
 	ErrValidationWrongSupplement = newValidationError("unexpected supplemental data")
+	// ErrValidationNotRelevant signals that a message is valid but not relevant at the current instance,
+	// and is not worth propagating to others.
+	ErrValidationNotRelevant = newValidationError("message is valid but not relevant")
 
 	// ErrReceivedWrongInstance signals that a message is received with mismatching instance ID.
 	ErrReceivedWrongInstance = errors.New("received message for wrong instance")

gpbft/errors_test.go

@@ -17,6 +17,7 @@ func TestValidationError_SentinelValues(t *testing.T) {
 		{name: "ErrValidationInvalid", subject: ErrValidationInvalid},
 		{name: "ErrValidationWrongBase", subject: ErrValidationWrongBase},
 		{name: "ErrValidationWrongSupplement", subject: ErrValidationWrongSupplement},
+		{name: "ErrValidationNotRelevant", subject: ErrValidationNotRelevant},
 	}
 	for _, test := range tests {
 		t.Run(test.name, func(t *testing.T) {