Create jBPM implementations of the WorkItemManager

The issue is that the queue-based execution model requires operations that happen after a "signalEvent" to be also added to the queue – otherwise, because an (internal) signal event is just an action in the queue, the operations after the (internal) signal event can cause queue actions after the signal event to not run properly.

This is the code that is causing the problem:

https://github.com/droolsjbpm/drools/blob/master/drools-persistence-jpa/src/main/java/org/drools/persistence/jpa/processinstance/JPAWorkItemManager.java#L146-L152
(and the same for abortWorkItem() )

https://github.com/droolsjbpm/drools/blob/master/drools-core/src/main/java/org/drools/core/process/instance/impl/DefaultWorkItemManager.java#L131-L134
(and the same for abortWorkItem() )

In short, I need jBPM specific implementations of the WorkItemManager in order to be able add jbpm-specific code to the WorkItemManager implementations.