[#378] metabase: Implement iterator over metabase graveyard

Implement `DB.IterateOverGraveyard` method that iterates over all graves and
passes passes their descriptors (new type `Grave`) to handler (new type
`GraveHandler`). `Grave` currently have buried object address and garbage
flag.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/local_object_storage/metabase/graveyard_test.go

@@ -0,0 +1,66 @@
+package meta_test
+
+import (
+	"testing"
+
+	"github.com/nspcc-dev/neofs-api-go/pkg/object"
+	meta "github.com/nspcc-dev/neofs-node/pkg/local_object_storage/metabase"
+	"github.com/stretchr/testify/require"
+)
+
+func TestDB_IterateOverGraveyard(t *testing.T) {
+	db := newDB(t)
+	defer releaseDB(db)
+
+	// generate and put 2 objects
+	obj1 := generateRawObject(t)
+	obj2 := generateRawObject(t)
+
+	var err error
+
+	err = putBig(db, obj1.Object())
+	require.NoError(t, err)
+
+	err = putBig(db, obj2.Object())
+	require.NoError(t, err)
+
+	inhumePrm := new(meta.InhumePrm)
+
+	// inhume with tombstone
+	addrTombstone := generateAddress()
+
+	_, err = db.Inhume(inhumePrm.
+		WithAddresses(obj1.Object().Address()).
+		WithTombstoneAddress(addrTombstone),
+	)
+	require.NoError(t, err)
+
+	// inhume with GC mark
+	_, err = db.Inhume(inhumePrm.
+		WithAddresses(obj2.Object().Address()).
+		WithGCMark(),
+	)
+
+	var (
+		counterAll         int
+		buriedTS, buriedGC []*object.Address
+	)
+
+	err = db.IterateOverGraveyard(func(g *meta.Grave) error {
+		if g.WithGCMark() {
+			buriedGC = append(buriedGC, g.Address())
+		} else {
+			buriedTS = append(buriedTS, g.Address())
+		}
+
+		counterAll++
+
+		return nil
+	})
+
+	require.NoError(t, err)
+
+	require.Equal(t, 2, counterAll)
+	require.Equal(t, []*object.Address{obj1.Object().Address()}, buriedTS)
+	require.Equal(t, []*object.Address{obj2.Object().Address()}, buriedGC)
+}