Improve gRPC connection state management

[na--]

https://community.k6.io/t/reconnect-in-grpc/1286 is a good example of how our gRPC implementation is not very flexible yet. Specifically, we don't have a good way to detect client disconnects. And I don't have a good idea how to handle them well, though something event-based seems reasonable, so we might need to implement #882 before we can...

[codebien]

I'm closing as this issue seems unlikely to be resolved.
If the client disconnects then I expect a re-run of the test will be required. The cases where the user wants to test the client disconnecting sound minor. And, in any case, with a bit of effort, the goal is still achievable by using the StatusDeadlineExceeded code as proved on the forum.

The current logic using the status code as proposed on the forum seems enough. We may re-evaluate the decision in the future if we find a higher demand.