`cin_delta` / `cout_delta` should be extended to 64bit

Hello,

Our FTTH stats show up as capped around 100Mbps. This is apparently simply coming from cin_delta and cout_delta being 32bit: with a report every 5 minutes, that's 4GiB/5minutes ~= 100Mbps.

cin_delta and cout_delta should thus be extended to 64bit integers, to handle accounting with nowadays bandwidths :)

That doesn't seem completely trivial since the sessiont structure is shared in clusters and a version number seems to be used to automatically convert, see cluster.c

Samuel