Fix: establish transaction and session before snapshot promotion/demotion

[treysp]

No description provided.

[erindru]

I was going to comment saying that this needed a test, but I think that updating the failing tests (which are failing because the new calls are unexpected) to now expect the calls should cover this

[erindru]

It looks like these are slightly different to the render_kwargs that _promote_snapshot() passes to model.render_session_properties.

For example, we pass snapshots here in _demote_snapshot() but not in _promote_snapshot(). And _promote_snapshot() gets a table_mapping property which is absent here.

Do you think these should be consistent so macros defined in session_properties can rely on the same things being present at both RuntimeStage.PROMOTING and RuntimeStage.DEMOTING?

[themisvaltinos]

in promote the table_mapping is mainly to be used for the on_virtual_update statements which are not relevant in demotion, but I do agree maybe they should be consistent across both, since we don't know what dynamic macros there might be in session_properties potentially