try to avoid passing around infinite values

[speezepearson]

• reject data points that contain NaN/infinite values
• if empty histograms are somehow encountered in the MeanAggregator, skip that data point entirely instead of creating a NaN value that will blow up down the road

Before:

$ curl http://localhost:8082/api/v1/datapoints -XPOST --data-binary '[{"name":"foo", "type":"histogram", "tags":{"a":"a"}, "datapoints":[[1500000000000, {"min":0, "max":1, "sum": 1, "mean": "Infinity", "bins":{"0":"1","1":"1"}}]]}]'; echo

$ curl http://localhost:8082/api/v1/datapoints/query -XPOST --data-binary '{"start_absolute":1500000000000, "end_absolute":1500000000000, "metrics":[{"name":"foo", "aggregators":[]}]}'; echo
{"errors":["org.json.JSONException: JSON does not allow non-finite numbers."]}

After:

$ curl http://localhost:8082/api/v1/datapoints -XPOST --data-binary '[{"name":"foo", "type":"histogram", "tags":{"a":"a"}, "datapoints":[[1500000000000, {"min":0, "max":1, "sum": 1, "mean": "Infinity", "bins":{"0":"1","1":"1"}}]]}]'; echo
{"errors":["mean has non-finite value Infinity"]}