Start a bridge's uptime history when it was first listed in a status
We're currently starting a relay's or bridge's uptime history when it was first listed in a network status with the Running flag. We do this to avoid displaying an uptime of 0% when the relay or bridge didn't even exist.
Turns out there are edge cases of bridges who are listed in the network status but without the Running flag. Those bridges should be displayed with an uptime of 0%.
We need to store an additional data point for these bridges: when they were first listed in a status. Our current data format doesn't support this, because we only store when a relay or bridge was listed as running.
We'll want to have this feature for relays, too, because we're going to parse votes at some point, and then we may have relays that are contained in some or even all votes, but with too few Running flags to also make it into the consensus.