Rxqueue spsc non-contended count proposal

Just a suggestion about making the current element count non-contended. I saw this trick in Disruptor / mechanicalsympathy if I remember correctly. Not benchmarked. 

(Also don't know how to aim a PR at @benjchristensen 's repository instead of main.)
According to [this](http://psy-lob-saw.blogspot.hu/2014/07/concurrent-bugs-size-matters.html) ```diff``` may become negative, but since ```count()``` is called by a drain method, that is either in the producer or consumer thread.