WorkItemNodeInstance#triggerCount is not persisted by AbstractProtobufProcessInstanceMarshaller

Hi, team

Seems like a bug (jbpm-flow project):

WorkItemNodeInstance provides an implementation of retry logic when ProcessWorkItemHandlerException is thrown. It relies on the data stored in two fields:  exceptionHandlingProcessInstanceId and retryCount.

The issue is that when the process instance is persisted and then restored the exceptionHandlingProcessInstanceId is restored correctly but retryCount is not persisted at all. Thus the retry logic (in WorkItemNodeInstance#processWorkItemHandler) doesn't work because the retryCount is set back to 0 on every iteration.