Currently, a ledger implementation can trigger backpressure by making WriteService#submitTransaction return Overloaded. This has the drawback that ledger can abort a command only after DAMLe has been executed. The current backpressure mechanism cannot be used to shield the system from excessive use of DAMLe.

To avoid that, extend ApiCommandSubmissionService with a callback isOverloaded that allows the ledger to tell if it is overloaded. Invoke isOverloaded before running DAMLe.