Too many #cellDiscarded messages

[darth-cheney]

As of this writing, there are two issues with sending #cellDiscarded messages that slow things down in a single rendering cycle:

1. The message has too much information, including the type of cell etc. It only needs to reference an id.
2. We are sending one message per each cell that is discarded in a rendering cycle. This gets to be grossly inefficient when we are, say, updating cells inside a giant table during one pass (could be thousands of separate messages).

Instead, we should restructure #cellDiscarded to instead be #cellsDiscarded which will simply pass an array of cell ids to remove per each cycle.