Add unsubscription messages in protocol

Two additional messages are added to protocol:
`unsubscribe_request_message` and `unsubscribe_reply_message`, used
to unsubscribe from a variable.
Make FV server accept unsubscription requests

The missing part reported in issue #32: `flight_vars_server` was not
processing unsubscription requests. This change introduces the behaviour
to process such kind of messages.