Feature/#574 look at httpclient cache key (#589)

* #574 consolidate some code, man the config stuff is a mess!

* #574 just use the downstream re route and the key for caching http clients

* #574 added benchmark, i was suprised to learn using a complex type was faster than a string in benchmark .net dictionary tests, hey ho probably dont have enough data in the type...

test/Ocelot.UnitTests/Authorization/AuthorisationMiddlewareTests.cs

@@ -16,6 +16,7 @@ namespace Ocelot.UnitTests.Authorization
     using Xunit;
     using Microsoft.AspNetCore.Http;
     using Ocelot.Configuration;
+    using Values;
 
     public class AuthorisationMiddlewareTests
     {
@@ -44,6 +45,7 @@ namespace Ocelot.UnitTests.Authorization
         {
             this.Given(x => x.GivenTheDownStreamRouteIs(new List<PlaceholderNameAndValue>(), 
                 new DownstreamReRouteBuilder()
+                    .WithUpstreamTemplatePattern(new UpstreamPathTemplateBuilder().Build())
                     .WithIsAuthorised(true)
                     .WithUpstreamHttpMethod(new List<string> { "Get" })
                     .Build()))