`NewClient` creates client storage, generates keys, connects, and registers with the network. Note that this does not register a username/identity, but merely creates a new cryptographic identity for adding such information at a later date.
`NewClientFromBackup` constructs a new client from an encrypted backup. The backup is decrypted using the `backupPassphrase`. On successful client creation, the function will return a JSON encoded list of the E2E partners contained in the backup and a json-encoded string containing parameters stored in the backup.