Expire pairing credentials

[leto]

It seems that pairing credentials do not expire. I connected SDA to SD after about 2 weeks, and it successfully connected with the same pairing code.

This might be seen as a feature, but it's a bug. If somebody steals your pairing code, they can use it indefinitely.

These pairing codes should expire after a short time. Very few users do anything but generate them and scan them a few minutes later. Expiring within 1 day should be reasonable, with potentially an advanced option to change it.