crypto/consensus: sign hashes and cache them for consensus payloads

Avoid serializing payload again and again for various purposes. To sign it, we
only need a hash.

Some 2.4% gain in TPS could be achieved with this.

pkg/core/interop_neo_test.go

@@ -216,7 +216,7 @@ func TestECDSAVerify(t *testing.T) {
 
 	t.Run("invalid message", func(t *testing.T) {
 		sign := priv.Sign(msg)
-		runCase(t, false, false, sign, priv.PublicKey().Bytes(),
+		runCase(t, true, false, sign, priv.PublicKey().Bytes(),
 			stackitem.NewArray([]stackitem.Item{stackitem.NewByteArray(msg)}))
 	})
 }