fix(engine): Distinguish new process instance from executions going from other executions from start node

[dbenesj]

Whet someone wants to implement it's own DefaultAuthorizationProvider.newProcessInstance() when new process instance is created, this method is called multiple times. Once for the process instance itself, but also for all executions (links) from the start event element defined in the process definition.
Reason is that when execution is being created (process instance is also an execution) it is first sent to be stored into the storage and only later process instance specific fields are being assigned to the java object. One of the reason is that ID of the execution is generating during the storing of the execution.
So to be able to distinguish the process instance itself from other executions while creating the process instance following steps has been performed:

1. Defined the placeholder to be used as a temporary process instance ID until actual identifier will be generated
2. Create new method that sets this placeholder to the Java object
3. Call the new method when creating new process instance instead of creating generic execution
   This way it ensures it is used only for the

Potentionally it is a question whether this new placeholder should be used also in ExecutionManager.createDefaultAuthorizations() to filter out other executions so that DefaultAuthorizationProvider.newProcessInstance() will be called really only for the process instance itself and not other executions...