fix #2870: renew tickets in HttpClient

fix #2870: renew tickets in HttpClient

by packing the auth into a RwLock and starting a background
future that renews the ticket every 15 minutes

we still use the BroadcastFuture for the first ticket and only
if that is finished we start the scheduled future

we have to store an abort handle for the renewal future and abort it when
the http client is dropped, so we do not request new tickets forever

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>