net: lwm2m: Prevent busy looping when active notify is set

In case a new notification is triggered while the previous one is still
active (i.e. not acknowledged), the LwM2M engine will start busy
looping, as the new notification is still valid (and supposed to be sent
after the previous one is done), but it's not sent in the current
iteration. The busy looping might prevent the system from receiving the
acknowledgment for the previous one, resulting in unresponsive system.

Fix this, by delaying the new notification timestamp a bit in such case,
so that the new notification will be scheduled for later instead of
trying to send it again right away.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>