Explicitly set control-group kill mode, that is: try SIGTERM first, and
use SIGKILL after a timeout. This exactly defines how bees is running as
a child process within the frontend service starter. Not sure if bees cares
about signals but SIGTERM first seems cleaner. On the way, let bees restart
on abnormal termination.
If you have a lot of or a few big nocow files (like vm images) which
contain a lot of potential deduplication candidates, bees becomes
incredibly slow running through a lot "invalid operation" exceptions.
Let's just skip over such files to get more bang for the buck. I did no
regression testing as this patch seems trivial (and I cannot imagine any
pitfalls either). The process progresses much faster for me now.