Old etcdserver which have not apply pr of #12165 will check auth token
even if the request is an Authenticate request.
If the client has a invalid auth token, it will not able to update it's
token, since the Authenticate has a invalid auth token.
This fix clear the auth token when encounter an ErrInvalidAuthToken to
talk with old version etcd servers.
Fix#12385 with #12165 and #12264