Add a retry queue to requests

[Justinzobel]

Quite often we'll get an error in our room saying

1 errors updating protected rooms!

    [Room Name] - Error during MatrixClient request GET [URL] - 503 Service Temporarily Unavailable -- 503 Service Temporarily Unavailable
    503 Service Temporarily Unavailable
    nginx
```

Mjolnir would be great if it could just keep trying since as it says this is a temporary service issue. The retry queue should be incremental of course, try after 5 seconds, then 15, 30, 60, 180, 600. Then if after all those tries it still cannot get the request in, send the error message to the channel.