Check behavior of invoice generate when user's clock is off by more than invoice expiry

[elsirion]

I was wondering about this recently, maybe we actually want to get consensus on the rough time (i.e. guardians are expected to have somewhat sane times set). Then both gateways and user clients could use the consensus time to decide if an invoice is still payable or can be considered timed out. Currently there might be a risk that the user considers an invoice that's still payable timed out if their system time is way off.