Batching events in Booster

This project comes from a question in the Booster's Discord channel. This code takes eventual consistence into account, taking advantage of Booster's default sharding capabilities to build batches of items on the fly without external transactional counters (which would limit the throughput significantly). It is intended only as a proof-of-concept. It hasn't been tested and should only be used at your own risk.

To solve this problem we do the following things:

1. The CreateItem command registers EventCreated events.
2. The EventCreated events are reduced and accumulated as Item instances in the ItemBuffer "singleton entity".
3. The EventCreated events also trigger the UpdateBatch event handler.
4. The UpdateBatch event handler checks the current state of the ItemBuffer singleton entity. If there's a completed batch, it register a BatchCreated and a CloseBuffer events.
5. The BatchCreated event creates a new Batch entity including all the items buffered to that point, which is then projected to the BatchReadModel and becomes readable via API.
6. The CloseBuffer event "resets" the buffer, deleting the completed batch and increase the batch counter.

Improvements and suggestions are more than welcome!