udev: remember UUIDs of triggered events

udev: remember UUIDs of triggered events

Otherwise, /run/udev/queue file may be removed before udevd receives
the triggered uevents. This avoids the following race:

1. uevent A is being processed,
2. received an inotify event and trigger a synthesized uevent,
3. uevent A has been processed,
4. the uevent queue becomes empty, and /run/udev/queue is removed,
5. received and queued the synthesized uevent, and /run/udev/queue is recreated.

With this patch, /run/udev/queue is not removed in step 4. Hence,
'udevadm settle' will wait for the synthesized event being processed.

This is similar to waiting for child processes that reread partitions
(thus trigger synthetic events) being terminated, but for synthetic
events triggered by the manager.