The current session refresh loop attempts to refresh access tokens when
they are due to expire in less than one minute. However, the code to
perform the refresh relies on a TokenSource from the x/oauth2 package,
which has its own internal 'expiryDelta' threshold, with a default of
10 seconds. As a result, the first four or five attempts to refresh a
particular access token will not actually refresh the token. The refresh
will happen only when the access token is within 10 seconds of expiring.
Instead, before we obtain a new TokenSource, first clear any existing
access token. This causes the TokenSource to consider the token invalid,
triggering a refresh. This should give the refresh loop more control
over when refreshes happen.
Consolidate this logic in a new Refresh() method in the oidc package.
Add unit tests for this new method.
