Rename type Contact

[IvanPizhenko]

This code makes me think we should start calling Contact something else. Probably DbPubkey, as the meaning of the structure has changed over time. That would be for another issue, of course.

Originally posted by @tomholub in #4003 (comment)

[tomholub]

closing - the goal will eventually be to get rid of the Contact type by refactoring places that use it.