Re-added support for CORS preflight requests.
Notification/command polling rewritten using async/await statements.
Command/notification polling intervals moved to configuration.
Re-fix bug when new devices could not be registered via WebSocket.
WebSocket actions framework was updated to pass the ping action.
Added Device.LastOnline field.
Added IDeviceReposetory.SetLastOnline() method which sets LastOnline to current time.
IDeviceReposetory.GetOfflineDevices() now uses LastOnline.
Updated device authentication attributes to update LastOnline for each request and WebSocket ping action.
Fixed null reference exception in erroneous situation.