Filters ist of post_id for new ones.

        :param source_id: id of the source
        :type string:
        :param post_ids: list of post ids
        :type list:
        :returns: list of unknown post ids.

async def filter_new_posts(self, source_id, post_ids):
        """Filters ist of post_id for new ones.

        :param source_id: id of the source
        :type string:
        :param post_ids: list of post ids
        :type list:
        :returns: list of unknown post ids."""
        new_ids = []
        try:
            db_client = self._db
            posts_in_db = await db_client.get_known_posts(source_id, post_ids)
            new_ids = [p for p in post_ids if p not in posts_in_db]
        except Exception as exc:
            logger.error("Error when filtering for new posts {} {}".format(source_id, post_ids))
            logger.exception(exc)
        return new_ids