the buildsystem will now pick up and link
all test cases according to the layer, e.g.
backend tests will automatically be linked
against the backend + library solely.
Meanwhile, BuffHanle became way more concrete,
making the separation of any kind of buffer or channel
type management concievable.
Thus: extract the obsolete ChannelDescriptor
and use switch at any location where the old
(superseeded) buffer handle is still referred
