Draft: chg: bridge: Filter out slower streams
when calculating bridges' ratio. In the same way this is done when calculation relays' ratio.
Also:
- Add fields to store the means and ratios
- Move out calculation of network means out of the bridge ratio calculation to stop doing it for every relay
- In case the scanner was cancelled before the latest consensus stored the bridge ratio threashold, use the the default in case this is None
Closes #139 (closed)