CSHARP-3810: Prevent pool pruning from acquiring a slot in _maxConnectionsQueue

Author: BorisDog

src/MongoDB.Driver.Core/Core/ConnectionPools/ExclusiveConnectionPool.Helpers.cs

@@ -745,7 +745,7 @@ public void Clear()
                 }
             }
 
-            public void Prune()
+            public void Prune(CancellationToken cancellationToken)
             {
                 PooledConnection[] expiredConnections;
                 lock (_lock)
@@ -755,6 +755,8 @@ public void Prune()
 
                 foreach (var connection in expiredConnections)
                 {
+                    cancellationToken.ThrowIfCancellationRequested();
+
                     lock (_lock)
                     {
                         // At this point connection is always expired and might be disposed

src/MongoDB.Driver.Core/Core/ConnectionPools/ExclusiveConnectionPool.cs

@@ -299,7 +299,7 @@ private async Task MaintainSizeAsync(CancellationToken cancellationToken)
                 {
                     try
                     {
-                        await PrunePoolAsync(cancellationToken).ConfigureAwait(false);
+                        _connectionHolder.Prune(cancellationToken);
                         await EnsureMinSizeAsync(cancellationToken).ConfigureAwait(false);
                     }
                     catch
@@ -315,20 +315,6 @@ private async Task MaintainSizeAsync(CancellationToken cancellationToken)
             }
         }
 
-        private async Task PrunePoolAsync(CancellationToken cancellationToken)
-        {
-            using (var poolAwaiter = _maxConnectionsQueue.CreateAwaiter())
-            {
-                var entered = await poolAwaiter.WaitSignaledAsync(TimeSpan.FromMilliseconds(20), cancellationToken).ConfigureAwait(false);
-                if (!entered)
-                {
-                    return;
-                }
-
-                _connectionHolder.Prune();
-            }
-        }
-
         private async Task EnsureMinSizeAsync(CancellationToken cancellationToken)
         {
             var minTimeout = TimeSpan.FromMilliseconds(20);