1.x: measure flatMap/concatMap performance when used as filter

[akarnokd]

This PR adds a perf class to measure the overhead of using empty() when the flatMap/concatMap emulates filter.

This will establish the comparison baseline for flatMap/concatMap optimization as requested in #1653.

The baseline numbers (i7 4770K, Windows 7 x64, Java 8u72):

Comparing flatMap against concatMap:

Here, mask indicates the and-mask that makes emitting empty less frequent: 0 = always, 1 = every other, 3 = every fourth, 7 = every eighth. This also means that rangeEmpty starts emitting more and more values with higher mask value hence the different throughput values.

[davidmoten]

Thanks @akarnokd, very good of you. I'll set up a PR to use it.