It takes too much of network/disk to handle tombstones expiration and removal,
so it is better to store it when tombstone is being indexed in metabase.
Additional little-endian expiration suffix was added to graveyard values.
Metabase version was increased as it is a non-compatible change. Relates #2929.

Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>

They do not differ from the other objects (e.g. locks do). Initial logic has
changed much, graveyard now allows to handle expired tombstones marks (do not
confuse it with the lists of regular indexes) independently, while disk can be
cleared with the other types of object. Also, add tests for it.

Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>

Graveyard now has tombstone expiration marks in epochs, there is no need to use
any network requests, just drop records if an epoch is big enough.
Closes #2929.

Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>

Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>

It may or may not index tombstone's expiration in graveyard.

Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>

Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>

Migration can be done automatically, let admin's life be a better thing. TS
expiration is taken as the current epoch + 5. It is not critical if TS will live
more. In practice, side effects _may_ be seen only the first 5 hours after an
update, and only returned status code _may_ differ.

Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>