To handle deletes and updates from chunk migration, we need to set up the parameter "fromMigrate" correctly. 

See https://github.com/10gen/mongo/blob/15a363b98be42d7dcc5db063185d143e0fa72b3c/src/mongo/db/exec/update_stage.cpp#L443 and https://github.com/10gen/mongo/blob/15a363b98be42d7dcc5db063185d143e0fa72b3c/src/mongo/db/exec/delete_stage.cpp#L261 for examples.