op-geth: don't error on receipt deserialization on startup (ethereum#34)

Part of the boot process looks at receipts in the database
to determine which schema is used. When booting with legacy
optimism style receipts in the database, an error message
is shown on boot. This error message doesn't do anything,
but this commit fixes the problem. If when checking for
legacy receipts and `true` is returned, then geth will
shutdown and tell the user to run a migration command
to migrate to newer style receipts. We return `false`
to prevent geth from shutting down. Further investigation
is needed to ensure that having these receipts in the
database won't cause any issues, we may need to migrate
them similarly to the tool that the geth team wrote to
upgrade the receipt serialization.

core/types/legacy.go

@@ -39,6 +39,10 @@ func IsLegacyStoredReceipts(raw []byte) (bool, error) {
 	if err := rlp.DecodeBytes(raw, &v5); err == nil {
 		return false, nil
 	}
+	var legacyOptimism []LegacyOptimismStoredReceiptRLP
+	if err := rlp.DecodeBytes(raw, &legacyOptimism); err == nil {
+		return false, nil
+	}
 	return false, errors.New("value is not a valid receipt encoding")
 }