Postfix 3.0 recently hit Debian Unstable (and Ubuntu Xenial for those that care about that). It’s been a bit of a bumpy road, but it seems to mostly be there for new installs. For package upgrades, there’s still issues. We hope to have that sorted shortly, but in the meantime, all you should need to do to get an upgraded system working is add or adjust two parameters in your main.cf
You can either edit the file directly or use postconf:
postconf -e shlib_directory=/usr/lib/postfix
postconf -e daemon_directory=/usr/lib/postfix/sbin
No need to file more bugs and yes, we also know postfix 3.1 was just released. One thing at a time.