Implement request status polling

[howjmay]

Currently, tangle-accelerator would distribute request id per request which would cause the client, an edge device, a burden to remember this information
Another proposal listed below will replace the current solution.

Client would generate a unique user-id by itself. The user-id may be generated from hardware information.
If client sent a request with user-id, then tangle-accelerator would track the status of this request with a redis list, and the status of the request will be updated in the list. If no user-id had been provided, then tangle-accelerator would return request ID as we are doing at this moment.

All the request status and response belongs a certain user-id will be responded with another API, request_status()

Another API for deleting the user-id may be needed