Auto-throttle the rate limit

[chfoo]

Support auto-throttling the rate limit to adjust for target server loads.

Examples:

• measuring the ping time to the server
• measuring the number of 500s
• checking whether connection to the server timed out within 5 seconds

This solution is the server counterpart of ArchiveTeam/seesaw-kit/issues/74

Edit: I realize I don't like the tracker making requests directly to the server because the tracker may be banned. Perhaps measuring implicit values such as:

• a derivative of the rate of times issued
• rate of data ingress
• a client reporting ping times
• number of claims