SymmetricNAT is never set back to false for a device

[russellb]

Once a device sets symmetric_nat to true in its Device record, it will never get set back to false once network conditions change again. I hit this for the Device record for my laptop, which gets moved around between different networks regularly.

To reproduce:

1. connect with nexd normally on a network without symmetric NAT
2. restart nexd with --relay-only to force the record to be updated with symmetric_nat: true
3. restart nexd again without --relay-only
4. Observe that the Device record still has symmetric_nat set to true. It should be false at this point.