task: mon_clock_skew_check: distinguish between on-going and finished check

Signed-off-by: Joao Eduardo Luis <jecluis@gmail.com>
task: mon_clock_skew_check: increase timeout and kick it off only on stop

We were kicking-off the timeout as soon as we started; it's better however
to kick if off only when we are told to stop (as long as 'at-least-once'
is true).

Signed-off-by: Joao Eduardo Luis <jecluis@gmail.com>