Fixes and improvements for SecureTcpChannel

[campersau]

Setting the socket property on Assign is required for shutting down the socket correctly and accessing the IsConnected property.

[campersau]

I also added a commit which surrounds reading and writing to the socket / stream with try catchs. So the channel does not go down if a tcp connection gets closed without the required termination handshake.

With all these fixes we survived a ddos attack last week. But unfortunatly the default BufferSliceCount of 100 was very low so it took a while until the server was fully recovered.