[#1318] engine: Change tombstone clear process

- Delete objects physically on tombstone's arrival;
- Store information about tombstones in the Graveyard;
- Clear Graveyard every epoch based on the information about TS in the
network.

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

pkg/local_object_storage/metabase/graveyard.go

@@ -2,10 +2,10 @@ package meta
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
 
 	addressSDK "github.com/nspcc-dev/neofs-sdk-go/object/address"
-	"github.com/pkg/errors"
 	"go.etcd.io/bbolt"
 )