ArduinoSession.Dispose() should also dispose its connection, if it owns it.

[jeroenwalter]

The SerialConnectionFinder returns ArduinoSessions instead of a serial connection.
The (serial) connection is no longer part of the interface, it is demoted to be an internal aspect of the ArduinoSession.
The rationale is that the serial connection typically only lives as long as the ArduinoSession. If you dispose of the ArduinoSession, the serial connection is also closed. Might as well dispose it then.

Also, SerialConnectionFinder should be renamed to FirmataFinder to emphasize the change in its interface of returning ArduinoSessions instead of serial connections.