Fix update_interval behaviour
Fix update_interval behaviour

Vhostmd uses the update_interval value to sleep() between the metrics updates.
But a long runtime of the metrics updates can result in a noticeable longer
time between the updates and a consumer might read 'old' data after waiting
update_interval seconds.
Fix the delay between updates, that
'runtime of metrics update' + 'sleep time' matches the update_interval value.

vhostmd/vhostmd,vhostmd/vhostmd,vhostmd/vhostmd,vhostmd/vhostmd