Wait for all requests in-progress to finish before shutdown

[andrewrabert]

Current
Upon SIGTERM, on_shutdown callbacks are executed.

Desire
Upon SIGTERM, stop accepting new requests and wait for in-progress requests to naturally finish. Only after all requests are done would the on_shutdown callbacks begin executing.

Why?
I typically disconnect database connections in an on_shutdown callback. This can result in errors when in-progress requests attempt to query said databases.