chg: bridgescan: Queue bridge when its desc arrives
instead of adding all known bridges to the queue to measure. Also:
- get tor bridge configuration before setting it, in order to do not set duplicated bridges
- when there's no progress measuring bridges, measure them again
- queue/measure bridges in smaller subsets to give time to the descriptors to arrive
- order the bridges to measure by those which doesn't have measurements first
- Create Bridge new field
_last_measured
to stop having to calculate it - Remove the code in
obtain_relays
about not getting last consensus when the last one was obtained less than 1h ago, what it's useful developing, but doesn't follow the same path and complicate the logic when getting the bridge descriptors
Closes #138 (closed)