Make sure only direct ByteBuffer are passed to the underlying jdk Channel.

This is needed because of otherwise the JDK itself will do an extra ByteBuffer copy with it's own pool implementation. Even worth it will be done
multiple times if the ByteBuffer is always only partial written. With this change the copy is done inside of netty using it's own allocator and
only be done one time in all cases.
Only copy buffer to direct buffer if pooled allocator is used
Add todo so we not forget
Address @trustin's comments