Merge branch 'zecke/channel-release'
Merge the channel release re-work. The most notable change is that
SAPI > 0 will now be released on the local end and that the release
process should be both more fast and more standard conform. With SMS
spamming/mass sending the nanoBTS is crashing but this does not appear
to originate from any of these changes. I used git rebase to go through
each of the change to see where the nanoBTS is crashing but couldn't
reproduce it. It might be a general overload or something generated by
the modems of the sysmocom modem bank. Merge it before the 29C3 so we
can test this code some more.