daemon: Support clean daemon shutdown

Instead of aborting the main loop as soon as a fatal signal (SIGTERM or
SIGINT) is received, additional logic allows waiting for tasks to finish
while I/O is still being processed.

If no callback function is provided the old behaviour--shutting down
on the first signal--is preserved.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>