Caching doesn't work for find queries in some configurations

[perederyaev]

Hello, I noticed that caching doesn't work for find queries for some configurations (for example when blacklist enabled). I've done some debugging and found that on the line https://github.com/go-graphite/graphite-clickhouse/blob/master/find/handler.go#L189 we try to call f.result.Bytes() which's not implemented in some wrappers (for blacklist - https://github.com/go-graphite/graphite-clickhouse/blob/master/finder/blacklist.go#L56). The error is skipped silently without any messages in log and caching is not working.