Python: record and expose FSM state changes

This MR records the time stamp of the last state transition and sends this information along with get_state/get_status commands (as meta information).

The format for the transmitted timestamp is what datetime.isoformat() provides: it is human-readable and easy to parse for other purposes.

Other changes included in this MR:

• python/core/cscp: Change CSCPMessage class variables to instance variables (as they are mutable types)
• python/core/cscp: use Optional where appropriate