BZ69355: ExactRateLimiter

[Chenjp]

replace #760: redo pr , change src branche from "main" to a dedicated branch "main_BZ69355". for headers section, compliance with draft spec of "https://datatracker.ietf.org/doc/draft-ietf-httpapi-ratelimit-headers ". comparing with no-ratelimit / or fast-ratelimiter, ab test of exact-ratelimiter performance shows that no significant throughout decrease (about -2% / -1%).