Function store_self_keypair 

pub(crate) async fn store_self_keypair(
    context: &Context,
    signed_secret_key: &SignedSecretKey,
) -> Result<()>

Stores own keypair in the database and sets it as a default.

Fails if we already have a key, so it is not possible to have more than one key for new setups. Existing setups may still have more than one key for compatibility.