Add a prefetch option to Optimistic DNS cache

[jumpsmm7]

I saw in the last release
Optimistic DNS cache (#2145)
To reduce latency you may make AdGuard Home respond from the cache even when the stored entry is expired, while trying to refresh them at the same timearrows_counterclockwise This checkbox is located in Settings → DNS settings → DNS cache configuration and it's not ticked by default. Responses made from DNS cache are marked with a special label in the Query log.

The incorporation of serving expired has been added for, in the same spirit of Optimistic DNS cache, would it be possible to add a prefetch policy ( if there is not one already incorporated that is?) to reduce how often entries are served as expired?

[agneevX]

I think it's better to serve expired entries with a very low TTL, and refresh the entry automatically in the background rather than prefetch.

However, it does make sense for popular items to be auto-refreshed on expiration.

[ainar-g]

Duplicate of #1259