Load balance right when we have higher guard rotation periods

Here's our plan:

1. Directory authorities need to track how much of the past n months each relay was around and had the Guard flag.
2. They vote a percentage for each relay in their vote, and the consensus has a new keyword on the w line so clients can learn how Guardy each relay has been.
3. Clients change their load balancing algorithm to consider how Guardy you've been, rather than just treating Guard status as binary (#8453 (moved)).
4. Raise the guard rotation period a lot (#8240 (moved)).