Problem: SIGTERM does not close the connection immediately

Right now, the program waits for a request to be made in order to break from the infinite loop. Needs a better implementation, like the suggestions mentioned in https://tokio.rs/tokio/topics/shutdown.

Relevant line of code:

https://github.com/laurenceisla/pgx_demo/blob/2ba2f84614beb00fcd870d7a0cf6e85200374e81/src/web_server_async.rs#L27-L29