Iterates through all connections which were previously listed as unavailable
        and marks any that have expired their retry_timeout as being up.

def check_down_connections(self):
        """
        Iterates through all connections which were previously listed as unavailable
        and marks any that have expired their retry_timeout as being up.
        """
        now = time.time()

        for db_num, marked_down_at in self._down_connections.items():
            if marked_down_at + self.retry_timeout <= now:
                self.mark_connection_up(db_num)