[master] DDoS mitigation features 3938. [func] Added quotas to be used in recursive resolvers that are under high query load for names in zones whose authoritative servers are nonresponsive or are experiencing a denial of service attack. - "fetches-per-server" limits the number of simultaneous queries that can be sent to any single authoritative server. The configured value is a starting point; it is automatically adjusted downward if the server is partially or completely non-responsive. The algorithm used to adjust the quota can be configured via the "fetch-quota-params" option. - "fetches-per-zone" limits the number of simultaneous queries that can be sent for names within a single domain. (Note: Unlike "fetches-per-server", this value is not self-tuning.) - New stats counters have been added to count queries spilled due to these quotas. See the ARM for details of these options. [RT #37125]