Add support for continuous scanning

[NullHypothesis]

In addition to one-off scans, it would be great to have support for continuous scanning. exitmap should then download the hourly consensus and have a smart way to scan new relays and re-scan existing relays.

[arlolra]

I'm working on something very similar here: https://github.com/arlolra/exitaddr

[NullHypothesis]

There are a bunch of things to balance here:

• We want to scan new exit relays soon but not always in the first hour, they are part of the network. That would make the scanner too predictable.
• We want to scan existing relays again. But not every hour. Perhaps once a day or every other day.
• We want to run every module over every relay but not immediately after each other as this would make the scanner's behaviour too predictable.

I'll start working on a branch.