net: lwm2m: Add option to force close the connection

Add an option to force close the LwM2M connection
instead of always trying to deregister.
If on a cellular connection and the connection is dropped,
deregistering will never complete and take a long time
before retries fail. This option allows the app to close the
socket and quickly re-establish the connection when the
network connection is available again.

Signed-off-by: Ryan Erickson <ryan.erickson@lairdconnect.com>