The expiration epoch is the _last_ valid epoch for an entity. Also, clarify
the expiration epoch meaning for tombstones and regular objects.
Signed-off-by: Pavel Karpy <p.karpy@yadro.com>
Sync lifetime names with `session` package. Add `AssertContainer` and
`AssertUser` methods which cover specific cases describe in protocol.
Increase test coverage.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
