Add basic JetStream backend.

[SpeedyCoder]

Though this might be a good thing to try out, as it looks to be super easy to deploy. Adding the substrate backend should make using it very easy. This implementation should cover basic usage, we can later add more configurability or do more involved things like allow partitioning using different subjects or wait for write acknowledgements on shutdown.

I've added additional test to check that subscriptions are indeed durable, as my initial implementation kept deleting them due to some odd behaviour in the JetStream library.