don't crash if bcache doesn't exists

if bcache doesnt't exists or loaded collectd-bcache crashes at start with message

bcache is not loaded.
Traceback (most recent call last):
  File "./collectd-bcache", line 139, in <module>
    main()
  File "./collectd-bcache", line 117, in main
    if len(uuids) == 0:
TypeError: object of type 'NoneType' has no len()
call sleep if bcache doesn't exist
print debug message as comment

as described here http://collectd.org/documentation/manpages/collectd-exec.5.shtml
each line beginning with a # (hash mark) is ignored.
flush buffer before sleep instead of disable buffering at all
fix for multiple bcache devices

when system has only one bcacheN device script worked fine, but when it has
more device stats from only one device was collected