ByteBufferSlicePool minor stuff.

I exposed the key for the property that limits cache queue size for each thread.
Undertow seems to use the default value constructor so using this system property seems the right way to use a bigger cache size.
Should all keys for properties that start with xnio be located in a single file?
Searching for "xnio. I found other entries that are not publicly exposed.
Thank you for your time.