Performance and Memory footprint analysis

[toefel18]

ImmutableStatisticalDistribution.java is threadsafe but creates a new object for every added sample or duration. The objects are immutable and consist of primitive fields only, making it easier for the garbage collector to make assumptions. The impact on performance and memory was therefore assumed to be low. This issue tests that assumption and the tests should be re-used when implementing a mutable version of StatisticalDistribution.