[aikins01]

telemetry login replaced the saved sign-in but left its refresh token valid at telemetry.dev, so each repeat sign-in left another working token behind. Now, once a new sign-in is saved, the CLI asks the server to revoke the token it replaced, the same way logout does. A cancelled or failed sign-in revokes nothing, and a failed revocation doesn't fail the sign-in.

The old sign-in is read under the same lock as the write that replaces it. telemetry.dev treats revoking a refresh token that another command already rotated as token reuse, and then revokes every token the client holds for that person, including the sign-in that just finished. Reading the old sign-in before taking the lock left a window for that. A refresh that is still in flight while the sign-in completes can still hit it; the person would then need to run telemetry login again.

Revocation and logout now share one function, which discovers the revocation endpoint each time instead of trusting a saved one.

Closes #115.