Simplify loopback NATS client.

Only use one goroutine per client instead of one per subscription.
This ensures that (like with the "real" client), all messages are
processed in order across different subscriptions.

natsclient_test.go

@@ -90,7 +90,7 @@ func testNatsClient_Subscribe(t *testing.T, client NatsClient) {
 		}
 
 		// Allow NATS goroutines to process messages.
-		time.Sleep(time.Millisecond)
+		time.Sleep(10 * time.Millisecond)
 	}
 	<-ch