Improve performance when loading a subset of tags with stats

[acelaya]

After a long time with hard to solve performance issues in the tags with stats endpoint, this has been drastically improved thanks to the introduction of the short_urls_visits_counts table (see #2074 and #2078).

However, the query is still complex, requiring a native query builder, and performance does not improve a lot when fetching only a subset of tags, compared to fetching the whole list.

The fact that fetching the whole list is now quite fast, makes this a non-critical issue, but it is a bit unintuitive that fetching the whole list and paginating client-side is still better than paginating server-side.

It would be useful to put some though on this and try to improve that query.