This allows to use an overridden `stop()` method when the Locust
runner stops the user, which allows for more flexibility without
introducing additional complexity.

One of the use cases is being able to close a gRPC channel before
killing the user greenlet.

Due to two main reason: (1) grpcio needs to be "patched" to use `gevent`
and (2) the existing patch has a bug (see
(grpc#15880)[grpc/grpc#15880]) implementing
gRPC load tests using Locust is not immediately obvious.

This example showcases how to implement a simple load test against
a gPRC server making sure that the requests are asynchronous.