Remove code-duplication from unsafe ByteBuf implementations.

Motivation:

We had a lot of code-duplication in our unsafe ByteBuf implementations which make it really hard to maintain in the long term.

Modifications:

- Introduce UnsafeDirectByteBufUtil which contains all the logic to use Unsafe on
- Make use of UnsafeDirectByteBufUtil in all our unsafe ByteBuf implementations

Result:

Share a lot of code and remove duplications. This will lead to easier maintance in the long term