MEDIUM: compression: start to move to the new buffer API

MEDIUM: compression: start to move to the new buffer API

This part is tricky, it passes a channel where we used to have a buffer,
in order to reduce the API changes during the big switch. This way all
the channel's wrappers to distinguish between input and output are
available. It also makes sense given that the compression applies on
a channel since it's in the forwarding path.