Invoke the callback in separate threads

	The callbacks need to be invoked in different threads to avoid clogging
	the IO pool.

	Fixes bug #639656.

mcs/class/System/System.Net.Sockets/Socket_2_1.cs

@@ -252,7 +252,12 @@ namespace System.Net.Sockets {
 				}
 
 				if (callback != null)
-					callback (this);
+					ThreadPool.QueueUserWorkItem (CB, null);
+			}
+
+			void CB (object unused)
+			{
+				callback (this);
 			}
 
 			SocketAsyncCall GetDelegate (Worker worker, SocketOperation op)

mcs/class/System/System.Net/WebAsyncResult.cs

@@ -139,9 +139,14 @@ namespace System.Net
 		{
 			if (!callbackDone && cb != null) {
 				callbackDone = true;
-				cb (this);
+				ThreadPool.QueueUserWorkItem (CB, null);
 			}
 		}
+
+		void CB (object unused)
+		{
+			cb (this);
+		}
 		
 		internal void WaitUntilComplete ()
 		{