when a message in nacked (e.g. 25), we add it to
nax.sink and create a %naxplanation message
(e.g. 1; first ever naxplanation sent) that contains the
nacked sequence number (25), and send it to the
original sender on the naxplanation flow.

when the ack for the naxplanation (1) comes back, we
know that this was an ack for a naxplanation bone
and then drop the sequence number of _this_
naxplanation message (1) instead of the original
message (25) that was nacked.

this causes a range messages in nax.sink
that will remain there until naxplanations catch
up with the actual message numbers from the
reference flow.

we are removing messages that might not have been
nacked, but that seems ok since the message won't be
there anyway.

(note: wip, currently does not work)

This reverts commit 05e1da9.