Add ByteBuf implementation optimized to wrap two ByteBuf [#2300]

Hi,

this is a first version of a ByteBuf optimized to wrap two ByteBufs (see  #2300). It's not done yet, but I think it's in a good enough state to get some feedback.

I also did some microbenchmarks on jdk8 hotspot on linux and for small bits of data (up to ~8KB) the performance gain seems quite significant: http://1drv.ms/1qPOUbq and https://gist.github.com/jakobbuchgraber/10548901. I ran the same benchmark on Windows Server and the results were similar, however I don't have them anymore (IT'S TRUE, I SWEAR! :P). I haven't done any integration tests with a real netty app yet.

WDYT @trustin @normanmaurer