token refresh improvements

[wjordan]

Two changes to improve token-refresh behavior in the client:

1. First try a Tokens.update without the UserURLCallback option (which can now fetch tokens in parallel), and then retry with the UserURLCallback option only if the first attempt returned any errors.
2. Adjust the (now-configurable) advancePrune duration (from the default 1 minute) so the foreground thread has an advance-prune duration of 30 seconds while the background thread has 2 minutes. This should make it more likely for tokens to be refreshed in the background rather than delay foreground operations.