Add a close() method to the PostgreSQLOnlineStore

Is your feature request related to a problem? Please describe.
When running the online store as a web app, I want to make sure that the connections made to the Postgres database using the PostgreSQLOnlineStore class are terminated gracefully.

Describe the solution you'd like
A close() method on the PostgreSQLOnlineStore which closes all open connections and connection pools.

Describe alternatives you've considered

Additional context