conversationrepository: pin certificates if not already in cert store

An account can try to connect to a device that the presence was never
yet detected on the DHT but can use the public key from the repository.
Bootstrap can use devices from the repository.
Moreover, do not try to bootstrap with expired devices, as they are
invalid anyway, so the connection will obviously fail.

Change-Id: I5493419681e88fa3f920efabb0ab7019319c05fd