Switch serenity's ratelimiter to a ratelimiting queue instead of returning a sleep duration

[tazz4843]

My bot sends large numbers of webhooks concurrently that hit Discord ratelimits quite quickly. Serenity's current implementation has them all fire simultaneously when the ratelimit expires, easily resulting in >10k 429s in under 10 minutes, and the dreaded 1 hour Cloudflare ban. I'm hoping this fixes the issue. Currently untested, opening for review.