Updated to Marten v4 alpha

Updated to Marten v4 alpha with revolutionary changes to Event Sourcing projections API.

More details in Pull Request: #38

See also:

• https://jeremydmiller.com/2021/02/09/event-sourcing-with-marten-v4-aggregated-projections/
• https://jeremydmiller.com/2021/03/02/improvements-to-event-sourcing-in-marten-v4/