improve no-data fallback mode for global ratelimiters

[Groxx]

bursts are surprisingly bad even in global mode, so this raises the burst in that scenario. when global usage is known, the unused-rps-boosting should take care of it, unless they are approaching their RPS (in which case reducing the burst is good).

(this is not yet complete, just proof-of-concept-ing for now and letting CI tell me if there are surprises)