Aggregations: Add an option to use HdrHistogram to compute percentiles

[jpountz]

HdrHistogram like t-digest can be used to estimate percentiles with a fixed amount of memory and has the interesting feature that two instances can be merged (useful for the reduce phase). It is less flexible than t-digest (assumes positive integers) and has a different take on accuracy (based on significant digits on the values, while t-digest gives more precision to extreme percentiles). That said, this approach makes sense to compute percentiles on response times, which is a common use-case and the specialization allows HdrHistogram to be very fast (I have not performed any benchmark but adding a new value with HdrHistogram is a O(1) while with t-digest it is a O(log(compression))).

[colings86]

It probably makes sense, as part of this change, to make the algorithm used pluggable in the same way that we have for the significance heuristic.