Close ``connection`` if specified, otherwise close all connections.

        Return a list of :class:`.Future` called back once the connection/s
        are closed.

def _close_connections(self, connection=None, timeout=5):
        """Close ``connection`` if specified, otherwise close all connections.

        Return a list of :class:`.Future` called back once the connection/s
        are closed.
        """
        all = []
        if connection:
            waiter = connection.event('connection_lost').waiter()
            if waiter:
                all.append(waiter)
                connection.close()
        else:
            connections = list(self._concurrent_connections)
            self._concurrent_connections = set()
            for connection in connections:
                waiter = connection.event('connection_lost').waiter()
                if waiter:
                    all.append(waiter)
                    connection.close()
        if all:
            self.logger.info('%s closing %d connections', self, len(all))
            return asyncio.wait(all, timeout=timeout, loop=self._loop)