better hotplug handling

This release should fix some crashes when hotplugging all over the place.

Known issues

With this release, just don't unplug the device during warmup :-)

• The logging thread is not stopped when the device is unplugged during warmup. Instead, the thread does not exit at all, which is a problem. It also cannot be stopped from the dbus interface, co2monitor has to be stopped manually (call the quit method on dbus, or do a sudo systemctl stop co2monitor).