[autoendpoint] Support unregister user endpoint

[AzureMarker]

Endpoint: DELETE /v1/{type}/{app_id}/registration/{uaid}
Documentation: https://autopush.readthedocs.io/en/latest/http.html#unregister-uaid-and-all-associated-channelid-subscriptions

Deletes the user and all channels.

Questions:

• Should this also delete all pending messages for the user in the message table?
  A: Yes. once a user is deleted, all data associated with the user should also be deleted.
  [autoendpoint] Support unregister user endpoint #179 (comment)

[jrconlin]

A: Yes. once a user is deleted, all data associated with the user should also be deleted.

[AzureMarker]

It turns out that in order to delete all of the user's rows in the message table, we would need to scan for them and batch delete them (due to the Hash + Range key):
https://stackoverflow.com/questions/34259358/dynamodb-delete-all-items-having-same-hash-key

The Python code simply deletes the row in the router table (not touching the message table). Should we continue to use this behavior or follow the suggestion from the StackOverflow post?