`Client.dial` can be called multiple times. For example, from regular
`NewClient` and [from the `Maintenance`
wrapper](6d451ab61d/client/v3/maintenance.go (L81-L84)).
This ends up creating two (if `Client.dial` is called twice) independent
`credentials.Bundle` instances:
- the first one is wired into `PerRPCCredentials` callback in gRPC
client.
- the second one is in `Client.authTokenBundle` and receives the latest
auth token updates.
When using username/password authentication and the server is configured
to issue JWTs, token rotation logic breaks because of the above
`credentials.Bundle` confusion.