Decrease per request memory pressure substantially by lazy voidReader creation.

Heya Jeanfrancois

we did some flight recording with the Java Mission Control and would like to optimize the creation of the ```voidReader``` in ```AtmosphereRequest```objects in order to decrease memory pressure.

One open point with the pull request is that I am not sure if the ```voidReader``` after having been marked as ```final``` originally, needs to be marked as ```volatile``` now, or visibility is guaranteed by the flow of execution (or the ```voidReader``` field is only subject of being accessed by a single thread anyhow). You are the master of the threading model :) !

Cheers, Christian.