image: Add bits and pieces for cgroups image

The exact structure of the image will be revealed in the
next patch(es). What is important here, is that cgroup
image is somewhat new.

It will likely contain arrays of objects of different types,
so I introduce the "header" object, that will link these
arrays using pb repeated fields. This will help us to avoid
many image files for different cgroup objects and will make
the amount of write()-s required be 1.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>