Save changes in an order

[jeffa5]

When changes are saved individually they currently might be read in in a random order. When this happens the loading can cause automerge to build up a large queue waiting for causal dependencies.

Storing them in some order, perhaps by actorid/seq might help us iterate through them in a nicer order. Even better would be to associate some sort of incrementing counter with the changes so we can sort by that.