Parse a (name,value) tuple from 'name value-length value'.

def _name_value(self, s):
        """Parse a (name,value) tuple from 'name value-length value'."""
        parts = s.split(b' ', 2)
        name = parts[0]
        if len(parts) == 1:
            value = None
        else:
            size = int(parts[1])
            value = parts[2]
            still_to_read = size - len(value)
            if still_to_read > 0:
                read_bytes = self.read_bytes(still_to_read)
                value += b'\n' + read_bytes[:still_to_read - 1]
        return (name, value)