Add custom ServerCtx with Release() method

Simplifies the server API by moving the release() function into a special context object ServerCtx.

Avoid reusing mutex for multiple streams.

Fixed Correctable streams.

incompatible changes

requestHandler: changed from func(context.Context, *Message, chan<- *Message) to func(ServerCtx, *Message, chan<- *Message)

compatible changes

(*Node).Host: added
SendMessage: added
ServerCtx: added