Transaction support
Created originally on Bitbucket by cdevienne (Christophe de Vienne)
The idea is that we may be able to provide a kind of process-level transaction.
An extra step after ProcessingEnd from the consumer would allow the server to validate the process transaction, and the consumer would then commit the transaction on its side.
The transaction would be closed only when the process has completed all the graph, and all the consumers sent a ProcessingEnd.
There would of course be a 'rollback' operation, allowing to cancel a process and make sure the consumers knows about it and take any necessary measure.