httpclient, handler build up and interceptors

More of a placeholder to come back to but I can't help but think the new HttpClient work which is being done will make the interceptors, httpclient creation/consumption and how the "pipeline" is done simpler. Not looked into it enough at the moment though.

https://blogs.msdn.microsoft.com/webdev/2018/02/28/asp-net-core-2-1-preview1-introducing-httpclient-factory/

Moving comment from #238 to close duplicates.

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Be interesting to see if using the new httpclientfactory - https://github.com/aspnet/HttpClientFactory/wiki/Consumption-Patterns - to use the library works