`Container` and `Object` types are transmitted in single `session.Token`
message. They differ only by session context.
Share common parts of the message in `commonData` struct. Embed struct
into `Container` and `Object`. Make `ReadFromV2` methods to check
protocol compliance. Make `Unmarshal`/`UmarshalJSON` to check field
format in case of presence only.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
