support swapping the para type

[drahnr]

Impl of swapping the para type from thread to chain and vice versa while keeping all modalities.

Ref #1509

[rphmeier]

The old behavior was something like this:

Two paras agree to swap. This can be chain<>chain, thread<>thread, or chain<>thread. Practically, we only really need to support the chain<>thread case. Then the swap is carried out: the chains' IDs stay the same, but their kind (thread/chain) are swapped, as well as their deposits.

Looking at this PR, it doesn't seem that this is what it's doing.

[drahnr]

No it doesn't, it was only the very beginning of swapping thread <> chain , but since then, #1559 appeared + assumptions got falsified.

[burdges]

I'd assume the swap only takes effect two epochs later or whatever?

[rphmeier]

@burdges At the end of the current session (epoch) was what I was thinking

[burdges]

It's fine I think. I worried about something that looks not really too problematic anyways.