Update LanguageServerHost to use NamedPipeServerStream

[beccamc]

Current communication between client and the Rolsyn language server is writing to STDOUT. This makes the connection vulnerable to any code in the process writing to stdout stream and corrupting the data.

We have seen at least one bug with this problem: microsoft/vscode-dotnettools#333

The fix it to use a named pipe to communicate.

• Client (e.g. VS Code) will kick off this server process
• Create the NamedPipeServerStream. Specify CurrentUserOnly flag , which indicates that the pipe can only be connected to a client created by the same user.
• Pass the name of the pipe back to the client, who will connect
• When the client connects, send the stream objects to Json-RPC

A note about names...

• Unix-type systems use a file name to create the connection.
• Windows systems use a pipe name. The format for the name is different in .NET code and Nodejs code.

This has been tested on both windows and mac.

[dibarbet]

mostly looks good! a couple of hopefully simple comments