Feature/poll consul (#392)

* WIP - implement a consul service discovery poller, lots of shared code with existing, refactor next and a todo in the docs to finish

* #374 implement polling for consul as option

* #374 updated docs to remove todo

* #374 fixed failing unit test

* #374 fixed failing unit test

* #374 fixed failing acceptance test

test/Ocelot.UnitTests/ServiceDiscovery/PollingConsulServiceDiscoveryProviderTests.cs

@@ -0,0 +1,89 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using Consul;
+using Microsoft.AspNetCore.Builder;
+using Microsoft.AspNetCore.Hosting;
+using Microsoft.AspNetCore.Http;
+using Moq;
+using Ocelot.Infrastructure.Consul;
+using Ocelot.Logging;
+using Ocelot.ServiceDiscovery.Configuration;
+using Ocelot.ServiceDiscovery.Providers;
+using Ocelot.Values;
+using Xunit;
+using TestStack.BDDfy;
+using Shouldly;
+using static Ocelot.Infrastructure.Wait;
+
+namespace Ocelot.UnitTests.ServiceDiscovery
+{
+    public class PollingConsulServiceDiscoveryProviderTests
+    {
+        private readonly int _delay;
+        private PollingConsulServiceDiscoveryProvider _provider;
+        private readonly string _serviceName;
+        private List<Service> _services;
+        private readonly Mock<IOcelotLoggerFactory> _factory;
+        private readonly Mock<IOcelotLogger> _logger;
+        private Mock<IServiceDiscoveryProvider> _consulServiceDiscoveryProvider;
+        private List<Service> _result;
+
+        public PollingConsulServiceDiscoveryProviderTests()
+        {
+            _services = new List<Service>();
+            _delay = 1;
+            _factory = new Mock<IOcelotLoggerFactory>();
+            _logger = new Mock<IOcelotLogger>();
+            _factory.Setup(x => x.CreateLogger<PollingConsulServiceDiscoveryProvider>()).Returns(_logger.Object);
+            _consulServiceDiscoveryProvider = new Mock<IServiceDiscoveryProvider>();
+        }
+
+        [Fact]
+        public void should_return_service_from_consul()
+        {
+            var service = new Service("", new ServiceHostAndPort("", 0), "", "", new List<string>());
+
+            this.Given(x => GivenConsulReturns(service))
+                .When(x => WhenIGetTheServices(1))
+                .Then(x => ThenTheCountIs(1))
+                .BDDfy();
+        }
+
+        private void GivenConsulReturns(Service service)
+        {
+            _services.Add(service);
+            _consulServiceDiscoveryProvider.Setup(x => x.Get()).ReturnsAsync(_services);
+        }
+
+        private void ThenTheCountIs(int count)
+        {
+            _result.Count.ShouldBe(count);
+        }
+
+        private void WhenIGetTheServices(int expected)
+        {
+            _provider = new PollingConsulServiceDiscoveryProvider(_delay, _serviceName, _factory.Object, _consulServiceDiscoveryProvider.Object);
+
+            var result = WaitFor(3000).Until(() => {
+                try
+                {
+                    _result = _provider.Get().GetAwaiter().GetResult();
+                    if(_result.Count == expected)
+                    {
+                        return true;
+                    }
+
+                    return false;
+                }
+                catch(Exception)
+                {
+                    return false;
+                }
+            });
+
+            result.ShouldBeTrue();
+        }
+    }
+}