The refresh_token used to refresh an operation_token.
This property does not need to be specified in order to perform token authentication with the driver. When specified, the driver will expect the user to manage and refresh the token as necessary. To have the driver manage and perform token authentication, set AuthScheme to Token, and supply the User and Password of a HarperDB user. The driver will retrieve, manage, and refresh tokens as necessary, encrypting and persisting the token credentials at CredentialsLocation.