anidb: Fix cache_test.go compilation
anidb: fix ed2k caching

Don't embed time.Time in the cache struct -- time.Time has its own
GobEncode and GobDecode methods, and we don't want them to be used.
anidb: Implement GroupByName

Also add the invalid key guard to GroupByID.
anidb: Wait longer after each UDP API timeout

Reset the wait after the first successful read.
anidb: Simplify documentation
anidb: Return the previously available data if API refresh failed

Makes the GetObjectByKey family of methods behave better if the API
is somehow unavailable.
anidb: Delete invalid cache data

If the API queries return errors specifically stating that requested
object doesn't exist, then delete the entry from the local cache.
anidb: Check the HTTP API error as well for deleting invalid Anime