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

Motivation:

Often protocols have messages that have two parts.
These are:
header
payload

For these protocols it would be pretty good if you could wrap two ByteBuf into one and and write it. The only way atm is the CompositeByteBuf which is just to slow because it allows to add/remove buffers on fly and so calculation of offset etc is expensive.

Beside this it also may be useful for our ByteToMessageDecoder as often only two ByteBufs are needed to cut-out a frame.

Modifications:

Add class BipartiteByteBuf

Result:

Faster than CompositeByteBuf
Addressed norman's comments
stupid