no concurrency support when using session cookies

[codecounselor]

Need to create a new (in-memory) session for every window that is created, otherwise concurrent requests will use the cookies applied to the single session created by the main process.

    // This creates a new session for every browser window, otherwise the same
    // default session is used from the main process which would break support
    // for concurrency
    // see http://electron.atom.io/docs/api/browser-window/#new-browserwindowoptions options.partition
    if( args.cookies ){
      defaultOpts.webPreferences.partition = uuid()
    }