Add BeginRead/BeginWrite/EndRead/EndWrite overloads back to SslStream…

… and underlying implementation. Unlike the default implementation in Stream it allows parallel read along with parallel write using the standard TaskToApm pattern (dotnet/runtime#33447). This reverts part of mono#17393 which could lead to deadlocks as reported in mono#18865.

mcs/class/System/Mono.Net.Security/MobileAuthenticatedStream.cs

@@ -338,6 +338,26 @@ public override Task WriteAsync (byte[] buffer, int offset, int count, Cancellat
 			return StartOperation (OperationType.Write, asyncRequest, cancellationToken);
 		}
 
+		public override IAsyncResult BeginRead (byte[] buffer, int offset, int count, AsyncCallback callback, object state)
+		{
+			return TaskToApm.Begin (ReadAsync (buffer, offset, count), callback, state);
+		}
+
+		public override int EndRead (IAsyncResult asyncResult)
+		{
+			return TaskToApm.End<int> (asyncResult);
+		}
+
+		public override IAsyncResult BeginWrite (byte[] buffer, int offset, int count, AsyncCallback callback, object state)
+		{
+			return TaskToApm.Begin (WriteAsync (buffer, offset, count), callback, state);
+		}
+
+		public override void EndWrite (IAsyncResult asyncResult)
+		{
+			TaskToApm.End (asyncResult);
+		}
+
 		public bool CanRenegotiate {
 			get {
 				CheckThrow (true);

mcs/class/System/System.Net.Security/SslStream.cs

@@ -488,6 +488,26 @@ public override Task WriteAsync (byte[] buffer, int offset, int count, Cancellat
 			return Impl.WriteAsync (buffer, offset, count, cancellationToken);
 		}
 
+		public override IAsyncResult BeginRead (byte[] buffer, int offset, int count, AsyncCallback asyncCallback, object asyncState)
+		{
+			return Impl.BeginRead (buffer, offset, count, asyncCallback, asyncState);
+		}
+
+		public override int EndRead (IAsyncResult asyncResult)
+		{
+			return Impl.EndRead (asyncResult);
+		}
+
+		public override IAsyncResult BeginWrite (byte[] buffer, int offset, int count, AsyncCallback asyncCallback, object asyncState)
+		{
+			return Impl.BeginWrite (buffer, offset, count, asyncCallback, asyncState);
+		}
+
+		public override void EndWrite (IAsyncResult asyncResult)
+		{
+			Impl.EndWrite (asyncResult);
+		}
+
 #else // !SECURITY_DEP
 		const string EXCEPTION_MESSAGE = "System.Net.Security.SslStream is not supported on the current platform.";