Condtionalty query scylla_tables for table metadata

When using scylladb/gocql with upstream Cassandra,
the driver queries system_schema.tables to get
table metadata.
When using scylladb/gocql with ScyllaDB, the
driver should also query system_schema.scylla_tables.

Closes #579

Author: ayashjorden

metadata_scylla.go

@@ -808,7 +808,8 @@ func getKeyspaceMetadata(session *Session, keyspaceName string) (*KeyspaceMetada
 	return keyspace, nil
 }
 
-// query for table metadata in the system_schema.tables and system_schema.scylla_tables
+// query for table metadata in the system_schema.tables, and system_schema.scylla_tables
+// if connected to ScyllaDB
 func getTableMetadata(session *Session, keyspaceName string) ([]TableMetadata, error) {
 	if !session.useSystemSchema {
 		return nil, nil
@@ -845,6 +846,10 @@ func getTableMetadata(session *Session, keyspaceName string) ([]TableMetadata, e
 		return nil, fmt.Errorf("error querying table schema: %v", err)
 	}
 
+	if !session.getConn().isScyllaConn() {
+		return tables, nil
+	}
+
 	stmt = `SELECT * FROM system_schema.scylla_tables WHERE keyspace_name = ? AND table_name = ?`
 	for i, t := range tables {
 		iter := session.control.querySystem(stmt, keyspaceName, t.Name)