Create only 1 callback instance for read/write

	Read/Write were creating one callback instance per call. Now we keep
	the same delegate around for all the calls.

mcs/class/System/System.Net/WebConnectionStream.cs

@@ -64,10 +64,12 @@ namespace System.Net
 		bool complete_request_written;
 		int read_timeout;
 		int write_timeout;
+		AsyncCallback cb_wrapper; // Calls to ReadCallbackWrapper or WriteCallbacWrapper
 
 		public WebConnectionStream (WebConnection cnc)
 		{
 			isRead = true;
+			cb_wrapper = new AsyncCallback (ReadCallbackWrapper);
 			pending = new ManualResetEvent (true);
 			this.request = cnc.Data.request;
 			read_timeout = request.ReadWriteTimeout;
@@ -95,6 +97,7 @@ namespace System.Net
 			read_timeout = request.ReadWriteTimeout;
 			write_timeout = read_timeout;
 			isRead = false;
+			cb_wrapper = new AsyncCallback (WriteCallbackWrapper);
 			this.cnc = cnc;
 			this.request = request;
 			allowBuffering = request.InternalAllowBuffering;
@@ -310,7 +313,7 @@ namespace System.Net
 
 		public override int Read (byte [] buffer, int offset, int size)
 		{
-			AsyncCallback cb = new AsyncCallback (ReadCallbackWrapper);
+			AsyncCallback cb = cb_wrapper;
 			WebAsyncResult res = (WebAsyncResult) BeginRead (buffer, offset, size, cb, null);
 			if (!res.IsCompleted && !res.WaitUntilComplete (ReadTimeout, false)) {
 				nextReadCalled = true;
@@ -365,7 +368,7 @@ namespace System.Net
 			}
 
 			if (cb != null)
-				cb = new AsyncCallback (ReadCallbackWrapper);
+				cb = cb_wrapper;
 
 			if (contentLength != Int32.MaxValue && contentLength - totalRead < size)
 				size = contentLength - totalRead;
@@ -509,7 +512,7 @@ namespace System.Net
 
 			AsyncCallback callback = null;
 			if (cb != null)
-				callback = new AsyncCallback (WriteCallbackWrapper);
+				callback = cb_wrapper;
 
 			if (sendChunked) {
 				WriteRequest ();
@@ -595,7 +598,7 @@ namespace System.Net
 		
 		public override void Write (byte [] buffer, int offset, int size)
 		{
-			AsyncCallback cb = new AsyncCallback (WriteCallbackWrapper);
+			AsyncCallback cb = cb_wrapper;
 			WebAsyncResult res = (WebAsyncResult) BeginWrite (buffer, offset, size, cb, null);
 			if (!res.IsCompleted && !res.WaitUntilComplete (WriteTimeout, false)) {
 				KillBuffer ();