Add new field last_running for "seen in a network status with the Running flag" in addition to last_seen for "seen in a network status"

There are two fields in relay details documents:

last_seen: UTC timestamp (YYYY-MM-DD hh:mm:ss) when this relay was last seen in a network status consensus. first_seen: UTC timestamp (YYYY-MM-DD hh:mm:ss) when this relay was first seen in a network status consensus.

And there are similar two fields in bridge details documents:

last_seen: UTC timestamp (YYYY-MM-DD hh:mm:ss) when this bridge was last seen in a bridge network status. first_seen: UTC timestamp (YYYY-MM-DD hh:mm:ss) when this bridge was first seen in a bridge network status.

Turns out that these definitions are confusing. We're not really interested in whether a relay or bridge was seen in a network status, but whether the directory authorities or the bridge authority thought it was running. So, whether the relay or bridge had the Running flag.

For relays this doesn't matter, because the consensus only contains running relays. (It will matter though once we add votes to Onionoo, because those include relays that don't have the Running flag.)

But for bridges it matters. Tonga includes all bridges in its network status that send it descriptors, including those without the Running flag.

Once we change the meaning of last_seen, we'll also stop giving out bridges that haven't been running for a week but that kept sending descriptors to Tonga. Should be fine.

This is going to fix #11410 (moved).