udisks2: Do not schedule update if pending already

Currently, the udisks2 volume monitor updates everything for each /proc/mounts, /etc/fstab, and udisks2 change. The update takes some time. If too many changes happen in a short timeframe, the udisks2 volume monitor is not able to handle those events in a real-time. The pending events can be processed for a long time. Many of those updates are redundant though as the real changes were already reflected during the previous updates. This causes a high CPU load among others. Let's simply not schedule a new update if there is a scheduled one already.

Fixes: #713