Auto flatten large documents

Currently, when a document state gets above one and a half megabytes, then edits are rejected and the document becomes impossible to edit. This is a rare but very annoying behavior of the system. At certain thresholds, we should auto-compact the document by throwing away edit history stored in state, this will allow the visible size of the document to grow much larger.

The caveat here is that if this is not done correctly, it will cause a merge of the old and new state to happen – having the opposite effect.