// RequestLimiter wraps http.Handler, limiting the number of
// concurrent requests being handled by the wrapped Handler. Requests
// that arrive when the handler is already at the specified
// RequestLimiter wraps http.Handler, limiting the number of
// concurrent requests being handled by the wrapped Handler. Requests
// that arrive when the handler is already at the specified
// Priority func is telling us to return 503
// immediately instead of queueing, regardless of
// queue size, if we can't handle the request
// Priority func is telling us to return 503
// immediately instead of queueing, regardless of
// queue size, if we can't handle the request