Onionperf loses all state at midnight

We have some experiments that keep or discard measurements based on the CBT state (whether the CBT is computed or not at the time of measurement). This is parsed from the logs at analysis time.

However, the logs are rotated at midnight. This means that if the CBT state changed in the logs for the previous day, this is not carried over - and measurements are discarded when they should not be. I've worked around this issue by making sure the instances which use guard rotation are started just after midnight, but a more robust idea is to keep some sort of state file for any values parsed from the logs that are used to filter measurements.