EdgeHub: Improve startup time

[varunpuranik]

• Parallelize EdgeHubConnection initialization tasks
• Reduce default OperationTimeout for all connections to 20 secs (from 1 min). This is for the following reasons -
  • For messages, this allows our retry logic to kick in faster instead of relying on the SDK
  • For twins, the C# SDK has a fixed timeout for Twin operations at 60 secs. So with the previous timeout of 60 Secs, the client would timeout before the EdgeHub, causing messages to be resent.
    https://github.com/Azure/azure-iot-sdk-csharp/blob/dad5ab9e1642c6a213e27bc1f5c09aebea7c9ac3/iothub/device/src/Transport/Mqtt/MqttTransportHandler.cs#L107
• This allows EdgeHub to start faster when offline, instead of appearing to hang, and run the risk of clients timing out