netifd: Rework hotplug event queueing in case of congestion

In case of event congestion (e.g. when several interfaces become active in a short notice) :
-hotplug events will be handled on a first come first served basis (before it was lifo)
-drop a new ifupdate event in favour of an already queued ifup event (before the ifup event was overwritten by the ifupdate event resulting in some hotplug scripts "missing" the ifup event)

Additonal the event flow has been documented

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>
Signed-off-by: Joeri Barbarien <joeri.barbarien@gmail.com>