Fix deadlock when connection is simultaneously aborted and ended (#684).

src/Microsoft.AspNetCore.Server.Kestrel/Http/Connection.cs

@@ -151,25 +151,26 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Http
         }
 
         public virtual void Abort()
-        {
-            lock (_stateLock)
-            {
-                if (_connectionState == ConnectionState.CreatingFrame)
-                {
-                    _connectionState = ConnectionState.ToDisconnect;
-                }
-                else
         {
             // Frame.Abort calls user code while this method is always
             // called from a libuv thread.
             System.Threading.ThreadPool.QueueUserWorkItem(state =>
             {
                 var connection = (Connection)state;
+
+                lock (connection._stateLock)
+                {
+                    if (connection._connectionState == ConnectionState.CreatingFrame)
+                    {
+                        connection._connectionState = ConnectionState.ToDisconnect;
+                    }
+                    else
+                    {
                         connection._frame.Abort();
+                    }
+                }
             }, this);
         }
-            }
-        }
 
         // Called on Libuv thread
         public virtual void OnSocketClosed()