Improve high-latency experience

[sylvinus]

High-latency scenarios are not well supported at the moment.

Some ideas:

• Add a COSR_ADDLATENCY environment variable to automatically add (random?) server-side latency. Would be helpful to test locally.
• Add a loading indicator after N ms waiting for a result
• Throttle ajax request better
• Make sure ajax responses are ignored if they arrive out-of-order