Simplify the multiversion handling in subnets

[babbageclunk]

Pull out some duplicated code. Put all of the version-dependent code in
one place - that should be the only part that needs to change when a new
version is added.

I think this is probably a good way to structure the multi-version code for other
types too - I'm going to use it the next time I need to do this.

[mjs]

This is certainly nicer. Thanks.

[babbageclunk]

$$merge$$