converting to use TDSessionDescription instead of OGCustomSessionDescription (client-side changes)

- TDSessionDescription is passed to server so that
  the fields (backupDirectory, serverGitRoot
  serverTodeHome) can be specified in the session
  description and used on the server
- for now server will request the session description
  to make upgrading easier (if client updated before
  server, server won't know what to do with the
  session description)