The remotewrite.Stop() expects that there are no pending calls to TryPush().
This means that the ingestionRateLimiter.Register() must be unblocked inside TryPush() when calling remotewrite.Stop().
Provide remotewrite.StopIngestionRateLimiter() function for unblocking the rate limiter before calling the remotewrite.Stop().
While at it, move the rate limiter into lib/ratelimiter package, since it has two users.
Also move the description of the feature to the correct place at docs/CHANGELOG.md.
Also cross-reference -remoteWrite.rateLimit and -maxIngestionRate command-line flags.
This is a follow-up for 02bccd1eb9
Updates https://github.com/VictoriaMetrics/VictoriaMetrics/pull/5900
