Fix string length in pack_string()

Specification says that strings are encoded as sequences of UTF-8
characters preceded by length in bytes. But the length was calculated
as a number of UTF-8 characters. So packing of strings with
non-ASCII characters was broken.

Some servers do not accept timestamps

Extended Node.set_value() to receive ua.DataValue objects directly.

Split on first ':' only.

If the name has a ':' in it then unpacking values fails, only split on
first.