net: lwm2m: Allow disabling server for a period of time

React to disable executable, as well as add callback that allows
disabling server for a period of time.

Also add API that would find a next server candidate based on the
priority and server being not-disabled.

Move all server related functions into its own header.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>