Improve concurrency handling in ExportAssetDownloader (#1002)

Co-authored-by: Oleksii Holub <1935960+Tyrrrz@users.noreply.github.com>

DiscordChatExporter.Cli.Tests/Infra/ExportWrapper.cs

@@ -9,6 +9,7 @@ using System.Threading;
 using System.Threading.Tasks;
 using AngleSharp.Dom;
 using AngleSharp.Html.Dom;
+using AsyncKeyedLock;
 using CliFx.Infrastructure;
 using DiscordChatExporter.Cli.Commands;
 using DiscordChatExporter.Cli.Tests.Utils;
@@ -20,7 +21,11 @@ namespace DiscordChatExporter.Cli.Tests.Infra;
 
 public static class ExportWrapper
 {
-    private static readonly ConcurrentDictionary<string, SemaphoreSlim> Locks = new(StringComparer.Ordinal);
+    private static readonly AsyncKeyedLocker<string> Locker = new(o =>
+    {
+        o.PoolSize = 20;
+        o.PoolInitialFill = 1;
+    });
 
     private static readonly string DirPath = Path.Combine(
         Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location) ?? Directory.GetCurrentDirectory(),
@@ -46,10 +51,7 @@ public static class ExportWrapper
         var filePath = Path.Combine(DirPath, fileName);
 
         // Lock separately for each channel and format
-        var channelLock = Locks.GetOrAdd(filePath, _ => new SemaphoreSlim(1, 1));
-        await channelLock.WaitAsync();
-
-        try
+        using (await Locker.LockAsync(filePath))
         {
             // Perform export only if it hasn't been done before
             if (!File.Exists(filePath))
@@ -65,10 +67,6 @@ public static class ExportWrapper
 
             return await File.ReadAllTextAsync(filePath);
         }
-        finally
-        {
-            channelLock.Release();
-        }
     }
 
     public static async ValueTask<IHtmlDocument> ExportAsHtmlAsync(Snowflake channelId) => Html.Parse(