Refactor Heroic Metric to add Distribution Support

[ao2017]

To improve percentile computation in heroic, we are introducing a data structure that preserves data distribution. Heroic currently supports only double data points. Double cannot be used for distribution support.

Code References:

Metric Implementation https://github.com/spotify/ffwd-client-java/blob/master/ffwd-client/src/main/java/com/spotify/ffwd/Metric.java

Protocol buffers used for serialization https://github.com/spotify/ffwd-client-java/blob/master/ffwd-client/src/main/proto/protocol0.proto

[ao2017]

PR : https://github.com/spotify/ffwd-client-java/pull/10/files