Atomically swap config data (dotnet/extensions#1202)

Swap the configuration providers' data atomically, rather than directly changing the property, so that any enumeration of the dictionary running during the reload operation does not throw an InvalidOperationException due to the collection being modified.
Relates to dotnet/extensions#1189.\n\nCommit migrated from dotnet/extensions@192abfd

src/Configuration.KeyPerFile/src/KeyPerFileConfigurationProvider.cs

@@ -31,12 +31,13 @@ private static string TrimNewLine(string value)
         /// </summary>
         public override void Load()
         {
-            Data = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
+            var data = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
 
             if (Source.FileProvider == null)
             {
                 if (Source.Optional)
                 {
+                    Data = data;
                     return;
                 }
 
@@ -61,10 +62,12 @@ public override void Load()
                 {
                     if (Source.IgnoreCondition == null || !Source.IgnoreCondition(file.Name))
                     {
-                        Data.Add(NormalizeKey(file.Name), TrimNewLine(streamReader.ReadToEnd()));
+                        data.Add(NormalizeKey(file.Name), TrimNewLine(streamReader.ReadToEnd()));
                     }
                 }
             }
+
+            Data = data;
         }
 
         private string GetDirectoryName()

src/Configuration.KeyPerFile/test/KeyPerFileTests.cs

@@ -6,6 +6,8 @@
 using System.Collections.Generic;
 using System.IO;
 using System.Text;
+using System.Threading;
+using System.Threading.Tasks;
 using Microsoft.Extensions.FileProviders;
 using Microsoft.Extensions.Primitives;
 using Xunit;
@@ -179,6 +181,47 @@ public void CanUnIgnoreDefaultFiles()
             Assert.Equal("SecretValue1", config["ignore.Secret1"]);
             Assert.Equal("SecretValue2", config["Secret2"]);
         }
+
+        [Fact]
+        public void BindingDoesNotThrowIfReloadedDuringBinding()
+        {
+            var testFileProvider = new TestFileProvider(
+                new TestFile("Number", "-2"),
+                new TestFile("Text", "Foo"));
+
+            var config = new ConfigurationBuilder()
+                .AddKeyPerFile(o => o.FileProvider = testFileProvider)
+                .Build();
+
+            MyOptions options = null;
+
+            using (var cts = new CancellationTokenSource(TimeSpan.FromMilliseconds(250)))
+            {
+                void ReloadLoop()
+                {
+                    while (!cts.IsCancellationRequested)
+                    {
+                        config.Reload();
+                    }
+                }
+
+                _ = Task.Run(ReloadLoop);
+
+                while (!cts.IsCancellationRequested)
+                {
+                    options = config.Get<MyOptions>();
+                }
+            }
+
+            Assert.Equal(-2, options.Number);
+            Assert.Equal("Foo", options.Text);
+        }
+
+        private sealed class MyOptions
+        {
+            public int Number { get; set; }
+            public string Text { get; set; }
+        }
     }
 
     class TestFileProvider : IFileProvider