Prevent concurrent conflicting write requests to the management model

The way the ModelControllers works makes it easy to support concurrent reads or reads occurring concurrently with writes. Reads get their own copy of the model; writes aren't reflected in the underlying model until the operation handler returns. But we need proper support for dealing with concurrent writes. Currently if two write execute concurrently, the "last one wins" i.e. its updated model gets written into the actual model, perhaps overwriting the changes made by earlier updates.