Add experimental liftbridge backend.

[SpeedyCoder]

[mjgarton]

This is a great start. Nice work.

Some of the missing bits are fine for now I think (partitions for example). I do think though that a large proportion of users expect durable subscriptions for their backends. (I realise that the "straw" backend does not have that though)

We could reflect the missing bits by explicitly documenting it, or we could mark as "incomplete" rather than "alpha"? (I'm now wondering whether a better approach overall to the implementation status would be a feature matrix. Maybe I will make a PR for that.)