Parachain/thread swapping

[rphmeier]

The old registrar removed in #1501 allowed the ability to swap parachains/threads. This kept the ID stable but swapped ownership of all deposits and removed any parathread scheduling.

The Paras and Scheduler modules need to be amended to support swapping. Swaps should occur only at session boundaries and only one swap per ParaId. It could be implemented by simply swapping within the Paras module and then making sure the Scheduler's initializer_on_new_session handler clears all entries from the ParathreadQueue which are not parachains - previously, this was not possible, but a thread<->chain swap makes it a case to account for.

[rphmeier]

This is addressed by #2406 to an extent?

[shawntabrizi]

From what I can read in the description of this issue, this should be fixed by #2406