ExponentiallyDecayingSample: change datastructure

* Use SortedList in reverse order instead of a ConcurrentDictionnary to
perserve sample order.
* Fix priority distribution according to the paper
* Always use UtcNow instead of Now for performance reason
Merge pull request #36 from razrin/pr/ExponentiallyDecaying

ExponentiallyDecayingSample: change datastructure