[Improvement] Asynchronous Interaction with the Database

[ScottCTD]

Initiations

• Now, our update-bill use case is very laggy.
• We need to find a way to make our program smoother.

Design

• In the UpdateBillUseCase, we can create 2 threads: The querier thread and the observer thread.
• The querier thread is responsible for querying entries from the database.
• The observer thread joins the querier thread. If the querier thread finishes its jobs, then the observer thread updates the GUI.