In this patch, we will make Safebrowsing V2 caching use the same algorithm as V4.
So we remove "mMissCache" for negative caching and TableFresness check for
positive caching.
But Safebrowsing V2 doesn't contain negative/positive cache duration information in
gethash response. So we hard-code a fixed value, 15 minutes, as cache duration.
In this way, we can sync the mechanism we handle caching for V2 and V4.
An extra effort for V2 here is that we need to manually record prefixes misses
because we won't get any response for those prefixes(implemented in
nsUrlClassifierLookupCallback::CacheMisses).
