Store registrar signature deadline in the Record

[peersky]

Currently we check registrar signature timeout during submit of the record.

In order to allow easier lookup on date validity, we want to add signature validity time to the record.

To do that, we need

• Add validUntil to LibMultipass.DomainStorage
• add validUntil to Record
• add public interface for validity verification `isValid(LibMultipass.NameQuery memory query) returns (bool)

Add tests:

• It returns correct validUntil parameter in user query
• It returns true for isValid if validUntil > msg.timestamp and false otherwise
• modifyUserName function requires validUntil to be longer than currently issued

[peersky]

completed in #6