Improve video transfer performance

[adamantivm]

Check if there are delays in the delivery of video when using the proxy server (particularly on Heroku) and think how to improve it

[adamantivm]

A few notes while testing today:

• Running with the proxy server on the same network doesn't show any signs of performance problems. The delay is very low (~ 1 second)
• NOTE: The image is throttled in the Turtlebot itself to about 5Hz, although it ends up being received in the web_video_server at about 2 to 3 Hz
• When running on Heroku the delay is very high but more importantly, the delay increases over time
• It is easy to change the resolution of the requested image by changing index.js in the web_teleop client code. This can further be tested easily by changing the code in a local proxy but connecting with it to the Proxy Server in Heroku

My conclusion is that either the bandwidth or the processing speed in Heroku is not enough to process the video and that causes an increasing delay over time. It would be nice to implement some sort of mechanism to drop frames if they take longer to send to the server than to receive them.

It is important to distinguish if the delay happens from proxy client to proxy server or from proxy server to web browser.

It is also important to distinguish if the problem is bandwidth or processing speed, since different solutions could be implemented depending on this.

[adamantivm]

More information: I think the main issue with this approach is that, when a video connection is open, the proxy client starts receiving data from the web_video_server and calls "connection.write_message()" for each chunk received from the video server. If the data is too much, Tornado in the proxy client takes too long to send each of those messages and they start getting queued for output. After a while, the proxy client has a huge list of messages to be sent which get older and older and older.