Connection for Connect causes Consul confusion on closing

When shutting down a task with a Connect sidecar, Nomad causes an error like this in Consul

```
2021-07-02T10:15:00.506-0500 [ERROR] agent.envoy: Error receiving new DeltaDiscoveryRequest; closing request channel: error="rpc error: code = Canceled desc = context canceled"
```

I suspect this has something to do with re-using a Context with `Context.Cancel` method being called to stop the connect which causes an error to be written over the wire to Consul.

You can see here it's impossible to call `Close` on the connection without the context already being cancelled. Since that context is shared when calling `DialContext`, I suspect the context cancellation error is being propagated over the wire.
https://github.com/hashicorp/nomad/blob/main/client/allocrunner/consul_grpc_sock_hook.go#L347

Start in client/allocrunner/consul_grpc_sock_hook.go around line 347 and trace how the shared context is passed to DialContext and then cancelled during Connect sidecar shutdown. Reproduce the shutdown and inspect the connection close path; done when closing the connection no longer sends the context-cancellation error to Consul.

Written by the indexing model from the issue text.