[3.2 -> 4.0] change HTTP code on exceedinghttp-max-bytes-in-flight-mb or http-max-in-flight-requests to 503

[spoonincode]

merge #2199 in to 4.0.

4xx response codes are for errors caused by the client where 5xx is for errors on the server side. The server exceeding its configured resource quota should be a 5xx error not a 4xx error. Change this from 429 to 503 (which is what it was in 2.0, fwiw). A 5xx error in this condition makes it easier for failover tooling, among other things. Resolves #2129

[ericpassmore]

Note:start
group: PROTOCOL
category: HTTP
summary: 503 is now the HTTP error return code for exceeding http-max-bytes-in-flight-mb or http-max-in-flight-requests. Previous code was 429.
Note: end