task: mon_clock_skew_check: mark as ran once if an expected skew was found

... even if we didn't get a clean/finished result from the monitors

This ought to significantly cut the waiting time if something else (or
someone else) is leaving the leader hanging thus unable to finish a given
timecheck round.

Signed-off-by: Joao Eduardo Luis <jecluis@gmail.com>
task: mon_clock_skew_check: use absolute value when comparing mon_skew

The monitors may report either positive or negative clock skews, and by
not using an absolute value we were constantly ignoring reported negative
clock skews.

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