Define common status code constants

[Tratcher]

Enum was always a poor type for Http status codes (e.g. System.Net.HttpStatusCode). Static/const ints may be more appropriate. We recently added the following for common header names: https://github.com/aspnet/HttpAbstractions/blob/dev/src/Microsoft.Net.Http.Headers/HeaderNames.cs. Something similar for status codes would be reasonable.