Use active bridge scan results to decide whether we can detect bridge blockings from passive bridge stats

We have a first version of a report titled Learning whether a Tor bridge is blocked by looking at its aggregate usage statistics. The main conclusion was that we were lacking actual bridge reachability data to conduct the analysis in a useful way. Once we have some results from active bridge reachability scans, we should revisit this report and write a second version.

This ticket is blocking on the active scanning data which is scheduled to be available by February 15, 2012. In that case the second version of this report shall be available by March 15, 2012, just in time for the sponsor E milestone. If the data doesn't materialize on time, there may not be a second version of the report on time.