new field: measured flag

Hi Karsten,

I would find it usefull to have a flag measured/unmeasured in relay details objects.

Use cases:

• make graphs based on how big is the fraction of unmeasured relays (-> detect spikes and BWAuth problems)
• Atlas would be able to display the flag, helping the operator with debugging

https://lists.torproject.org/pipermail/tor-relays/2015-May/006946.html