Ensure that onionperf ignores measurements and timeouts, when no timeout is learned yet, while changing guards

A new field, cbt_set, is added for every circuit; this is set to false at the beginning of a measurement, and gets set to True after a BUILDTIMEOUT_SET COMPUTED event. Each BUILDTIMEOUT_SET RESET event resets the field to False. Any measurements that specify --drop-guards will use this field to exclude circuits built when CBT was not computed from the daily analysis by default.

Analysis files for these measurements are also tagged by default with the 'exclude_cbt' filter, which is then used by the visualize command to only visualize results that used non-excluded circuits.