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. I haven't done any integration tests with a real netty app yet.

WDYT @trustin @normanmaurer