Fix OOIION-806/897: Process state fixes and enhancements

- Fix OOIION-897: Concurrent _flush_state in greenlets now ok through lock
- Fix OOIION-806: Process state now loaded on process restart
- Process state improved update performance. Eliminate one read
- Enhancements to state repository interface: Provide the ProcessState object context
- Polish of process state handling and state repository