Event sourcing | Can use read database within Command handler.

[hyunbin7303]

https://stackoverflow.com/questions/31386244/cqrs-event-sourcing-check-username-is-unique-or-not-from-eventstore-while-sendin

[hyunbin7303]

But it is not desired due to race condition / eventual consistency issue. So for this project, we won't use this approach as much as we can.