Race condition between adding contacts and adding transports
KeyManagerImpl#addContact() runs synchronously whereas
KeyManagerImpl#addTransport() runs asynchronously. If a transport is added and then a contact is added immediately afterwards, it's possible for the contact not to get any transport keys.
This is very unlikely to affect real users because transports are added early in the startup process, but it's causing SimplexTransportIntegrationTest to fail intermittently, and should be fixed anyway on principle.