Add support for Preemptive token refresh

The current implementation of token refresh doesn't take the expiry date into account, instead waiting until a request fails and only then refreshing the token.

It would be smoother for the refresh to be made in advance of expiration to reduce the use of expired tokens.

Start by tracing the existing token refresh implementation and how token expiry dates are represented and checked. Determine where refresh currently follows a failed request, then define and implement the advance-refresh behavior; done means valid tokens are refreshed before expiry without waiting for a request failure.

Written by the indexing model from the issue text.