nextUnusedAddress
Get the next unused address for the given keychain, i.e. the address with the lowest derivation index that hasn't been used in a transaction.
This will attempt to reveal a new address if all previously revealed addresses have been used, in which case the returned address will be the same as calling `Wallet::reveal_next_address`.
WARNING: To avoid address reuse you must persist the changes resulting from one or more calls to this method before closing the wallet. See `Wallet::reveal_next_address`.