The GIN type of search (using `&&` operator) is very slow. It appears
that we might have a queries in range of 1-10ms if we use a dedicated
index that decomposes traversal_ids and perform prefix search in form of:

```sql
SELECT * FROM ci_namespace_mirrors
WHERE ((traversal_ids[1])) IN ((1)) OR (((traversal_ids[1]), ((traversal_ids[2])) IN ((1,2))
```

The 4-levels are used since this appears to be enough to cover 99% cases queried
by the index. Beyond 4th level it will be more efficient to do seq scan of remaining
entries.

Changelog: performance