Make `node-syphon` client (data-sharing) ready for nodejs `worker_threads` or browser WebWorkers.

[benoitlahoz]

As of now, node-syphon requires hacks on user's javascript implementation side to work in parallel threads (workers).

Client: until we find a way to avoid downloading GPU texture to CPU data, we should refactor the way client is handled in the addon itself, by

• Providing single source of truth for a given server subscription, whatever how many subscribers are involved (currently done on javascript-side).
• Consequently handling the frame bufferization in a threadsafe kind of 'server store'.
• Transparently providing the javascript-side client with a shared and 'worker-ready' frame on 'frame' event listener.

As by doing this, we would... just mirror the way Syphon works, but in a kind of still hacky memory-sharing of pixels and not texture / IOSurface, I guess we could at least handle this pixels sharing in a threadsafe way so destination canvases, video encoders, etc., GPU to CPU download and pixels writing would not affect the application itself.