Document that `Version.ToV2` method return `nil`
when called on `nil`. Document that `NewVersionFromV2`
function return `nil` when called on `nil`. Write
corresponding unit tests.
Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>
Implement function that checks the compatibility of the specified version
with the current NeoFS revision.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
