Add MiddleOnly flag to RelayFlags
This flag is needed to implement tpo/network-health/margot#10 (closed).
At the moment, it will only be used to parse MiddleOnly from relays' network status documents, not to generate them.
This flag is needed to implement tpo/network-health/margot#10 (closed).
At the moment, it will only be used to parse MiddleOnly from relays' network status documents, not to generate them.