Configurable Overflow Per-Bag

[JonathanMace]

A bag could predefine the point at which overflow is acceptable, so that the bag itself can overflow without causing all subsequent bags to overflow

To achieve this, baggage buffers would add a way of annotating the acceptable size limit

Then in the baggage protocol, we would need a per-bag overflow marker. I think the best way of doing this would be to add an indexed child with no index specified (which would therefore be lexicographically less than the bag's other children)