carbonara: fix archive back window

We used a back window of 2 block sizes before, but we should by default
use a one block back window, meaning it's not possible to send data
backward. We now allow this back window to be configured to be any
timespan the user wants.

Change-Id: Ib934dc6f1cbce8e7595a20626a5d3a0b5b4678e6