LookupCacheV4::Has implements safebrowsing v4 caching logic.
1. Check if fullhash match any prefix in local database:
- If not, the URL is safe.
2. Check if prefix is in the cache(prefix is always the first 4-byte of
the fullhash, Bug 1323953):
- If not, send fullhash request
3. Check if fullhash is in the positive cache:
- If fullhash is found and it is not expired, the URL is not safe.
- If fullhash is found and it is expired, send fullhash request.
4. If fullhash is not found, check negative cache expired time:
- If negative cache time is not expired, the URL is safe.
- If negative cache time is expired, send fullhash request.
MozReview-Commit-ID: HFpqaOGOtUa
