Bug-612: Added ByteBufWriteUtil class

-ByteBuf's write methods utility - writes value if not null, otherwise zeros.

Change-Id: Ie3609df888e83dffb69bf9a5da78a3a1ca4ce91d
Signed-off-by: Milos Fabian <milfabia@cisco.com>
Merge "Bug-612: Added ByteBufWriteUtil class"