Assign HSDir flag by wfu and tk now too

Directory authorities now assign the HSDir flag (for serving onion service descriptors) in a way that aims to make it more expensive for new relays to quickly become part of the HSDir ring.

The old requirements were "has the Stable and Fast flags and has an uptime over 4 days." Now we keep the Stable+Fast requirement, reduce the uptime requirement to 1 day, and demand a threshold of weighted fractional uptime (the same wfu requirement as the Guard flag) and a threshold of time-known (must be in the top quartile of relays by time-known).

Resolves ticket 19162.