Fix message logging and filtering
When running in measurement mode, new analysis results are produced every night at midnight utc. However, in cases when OnionPerf was not running at midnight and restarts on a later day, log messages from both partial days appear in the same log file. When the analysis is run at the midnight following the day it was restarted, downloads from the day it was first stopped may appear in the analysis results (the .tpf files).
We should fix this.
allow onionperf analysis mode to filter out log messages that do not match a certain date; onionperf measurement mode should take advantage of that feature
When exporting Torperf .tpf files, and a date filter is applied, we need to ensure that we do not include downloads that started on a different day (to handle the case where it ended on the filtered date but started on the previous day)
onionperf should name the log files according to the date of the log message timestamps, and each log message should only be appended to the file corresponding to that date
(Copied from https://github.com/robgjansen/onionperf/issues/31)