More intelligent ovalues mismatch resolution

[grobian]

Currently zipper takes the first response received when there is a mismatch in granularity between copies received. This is due to configuration issue on the backends, but nevertheless it would be nice if the zipper could handle this more intelligently, such that we see more, and have less flapping (data is there, next load data isn't). For example, zipper could return the metric that has the least missing values (percentually?), iso the first received.

[dgryski]

I might only implement it for the JSON responses

[dgryski]

Also could return the highest granularity metric in case of conflicts.