Improve Refresh Token lifecycle management in Live Connect

For service like Box, the refresh token is valid until 60 days and could be used only once to get a new access token.

As the credential retrieving could be concurrent, two threads could ask an access token refresh with the same refresh token.
First one will work and the new access token and refresh token will be saved in DB. But as the second one won't work, values saved in DB at this time will be a null access token and the old refresh token (one which is invalid at this moment).