Implement semi-expiration

[JeremyRand]

Is your feature request related to a problem? Please describe.

Currently, semi-expiration is enforced by Electrum-NMC but not Namecoin Core. This means that the resolution behavior of Namecoin is different depending on which name_show provider is used. This decreases the effectiveness of semi-expiration as a name loss mitigation, and increases the risk of outages if a name owner fails to notice that their name is semi-expired.

Describe the solution you'd like

Implement semi-expiration in Namecoin Core's name_show method, with the same behavior as Electrum-NMC.

Describe alternatives you've considered

I cannot think of any alternatives.

Additional context

N/A.