Improve log summary code

The log summary function now looks specifically for deferred lines, which are more indicative of problems with remote mailserver configs. It also has a 'cron mode' that produces no output if everything is fine. Currently cron mode doesn't do anything with trusted/untrusted log lines, because we don't yet have any hosts configured for cert verification.

Future work: Add more useful detail to the error output, notify any endpoints specified in the starttls config.