net: lwm2m: Fix Security and Server object instance matching

A proper way to match a Security object instance with a Server object
instance is via Short Server ID resource. Both coupled object instances
should carry the same value of this resource in order to me considered
matched.

This was not implemented in the LwM2M library and it was incorrectly
assumed that the Security object instance index corresponds to the
Server object instance index. While such apporach works is simple
scenario, it might yield incorrect results when bootstrap is used.

Fix this, by verifyng the Short Server ID resource in the Secuirty
instance used, and finding a matching Server instance. The server object
instance is stored for future use in the engine.

Additionally, remove an extra Server object instance that was created
when the bootstrap procedure was used. Since the boostrap Security
object instance does not have the corresponding Server object, it's
enough to have a single Server instance.

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