addKey = async (id, key) => {
    const { privateKey } = key
    await storage.put('private_' + id, privateKey)
    // Unmarshal the key and add it to the cache
    const unmarshaledPrivateKey = privateKeyFromRaw(privateKey)
    await keyCache.put(id, unmarshaledPrivateKey)
  }
Adds a private key to the keystore.
@param {string} id An id of the [Identity]{@link module:Identities~Identity} to whom the key belongs to.
@param {Uint8Array} key The private key to store.
@memberof module:KeyStore~KeyStore
@async
@instance