Allow configuring DNS cache size

[ameshkov]

Currently, we're using the small default cache sizes. We should make them configurable instead.

• cache_size -- DNS cache size
• safebrowing_cache_size -- safebrowsing service cache size
• parental_cache_size -- parental control service cache size

Default values should be relatively small, I think 5000 for each should be enough.

[szolin]

we're using the small default cache sizes

Current DNS cache size is 1000 elements.
However, current safebrowing/parental/safesearch cache size is 64k elements.

[ameshkov]

However, current safebrowing/parental/safesearch cache size is 64k elements.

Yeah, and it makes little sense to have them that big by default:)

[skydrome]

can we get a setting to modify ttl values also? Or does defaultCacheTime ignore ttl?

[thueske]

+1

[ameshkov]

@skydrome yep, there will be a value for safebrowsing/parental caches TTL.