Multi-Step Key Generation

@r10s reports that some modern phones can take more than a minute to generate an RSA key using OpenSSL. To mitigate this, it should be possible using Sequoia's high-level API for an application like Delta Chat to launch a thread as soon as the device starts to generate a key without a user id while the user enters her account details. Then, once the account has been configured, finishing key creation should be a matter of creating a UserID.