Bring back `upgrade_downgrade_tests` after 0.8 is cut

[benthecarman]

In #943 we removed the upgrade_downgrade_tests because we broke this functionality with the pagination changes to our KV stores. We need to bring these tests back and test against downgrading to 0.8 once it is cut.

[big14way]

@benthecarman Mind if I take this one? I see it's assigned to you so feel free to keep it if you'd rather.

Just to check I follow: #943 swaps the build_with_store bound to PaginatedKVStore, so the old downgrade test won't compile since it wrote state through the legacy v1 store. And because the new paginated layout isn't readable by 0.7.0, the test only makes sense once there's a paginated version to downgrade to, which is 0.8.

So my plan would be to wait for 0.8 to be tagged, add an ldk-node-080 dev dependency, and bring the test back pointed at 0.8. Sound right?