jamiaccount: fix startSync race condition.

When receiving a new DeviceAnnoucement the other side will try to get the
device certificate. However we can't be sure that the certificate is on the
DHT. So sync will generally fail the first time.
This patch introduces a new callback to startSync only when the certificate
of the device is present on the DHT.

Change-Id: If2334051b1eea07d5036f50f095e5d64ef3ba660