Add creationTimeMillis as bean property to replace internal birthDate local variable

Birthdate is written/read to/from ObjectStore files without being available via API. Actually, it is only used for trace logging. I would suggest to:

1. Add creationTimeMillis as long property in StateManager class
2. Initialize with System.currentTimeMillis() on bean creation
3. Make creationTimeMillis available via getter method

Such an enhancement would make it possible to evaluate the "real" age of open transactions to e.g. improve ExpiryScanners.