Reconsider XON/XOFF PR to keep slave in sync with terminal

[Tyriar]

See if #146 performs better after #422 lands. It could also be changed to let larger chunks of data (~1 viewport page?) as that should suffice for keeping ctrl+c more responsive as well as making commands such as time accurate.

[Tyriar]

I think pausing the process once a certain threshold is hit so xterm.js can catch up is the best thing to do here. The threshold could be quite large as well, say 1000+ lines or something, it's just commands that send a tonne of output will always outrun us.

[parisk]

Should we consider implementing this eventually? I think that after #422 performance improved radically and I am quite skeptical about this as it seems more like a hack, rather an actual performance improvement.

[Tyriar]

This is a must imo after looking into perf over the last week. Running a command such as yes will send too much output for xterm.js to possibly handle, meaning it will be chugging as fast as it can all the time and interrupting such a process is only possible once the output has been processed. Definitely not hacky, it improves stability significantly in certain use cases in addition we support XON (ctrl+s) and XOFF (ctrl+q)!