Make placement `Traverser.Next` method to return ``[]network.AddressGroup`
in order to support multiple addresses of the storeage nodes.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
`network.Address` structure in most cases created once and used read-only.
Replace `AddressFromString` function with `Address.FromString` method with
the same purpose and implementation. Make all libraries to work with value.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
