net: lwm2m: Implement notified value registry for gt/lt/st attributes

Greater than, Less than and Step attributes require to track the last
notified value of a resource/resource instance in order to be able to
apply the specific thershold/step rules that the attributes define.

Therefore, implement a simple registry of the last notified values. When
one of the gt/lt/st attributes is configured on a resource/resource
instance, a registry entry is allocated for respective resource.
Whenever a notification is sent (either as a reply to Observe message,
or proactively) the registry is updated for the notified resources.

The stored resource values have been unified as "double" variables, to
avoid implementation complexity of having to support different
integer/floating point resource types.

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