perf optimizations for large stream notebook outputs

[amunger]

for #182636

• skip rendering outputs if we've already sent a message to the renderer for that version of output
• added an 'appended' field to the message object for text that was added since the previous showOutput message (not on the API at the moment)
• streaming output will be appended to a scrollable output element to reduce the amount of DOM manipulation
• adjusted the limits of lines in the scrollable outputs: if a large output comes in, display up to 5000 lines of it (unchanged) but keep appending output up to 8000 lines.
  • once the 8000 line limit is reached, replace it with the last 5000 lines of the full output. That way we can continue to append on top of that

[rebornix]

Great work, I absolutely love it!

[rebornix]

This is very smart