GraphQL Subscriptions and kick-off the stabilization phase

This release finishes the GraphQL functionality that was still missing: support for subscription operations.

Now you can subscribe to any of your read models, specifying any filter you want. After that, whenever you update that read model through any event, you will be notified with the updated version of the read model if it matches the filters you specified.
It works similarly to GraphQL live queries.

With this missing piece, Booster is now suitable for real-time apps!

As always, there were a lot of bugs fixed.

The project now starts a new phase where we focus less on adding features and more on stabilizing the public API, fixing bus and polishing any rough edge