add cache on the filters values

[agallou]

The request to get the mainteners list is sometimes slow :

SELECT package.maintainer FROM `package` GROUP BY package.maintainer ORDER BY package.maintainer ASC

https://github.com/agallou/mageia-app-db/blob/master/lib/filtering/filters/maintCriteriaFilter.class.php#L10

@stormi we may need to add a maintainers table

In the meantime, we could add cache on the filters value (there is cache on the filters components but it depends on the url parameters). Cache it on a redis/memcached.

This cache should be invalided after each synchronization.