[#128] metabase: Implement Delete method

Implement Delete method on DB structure that adds deleted addresses to
tombstone index. Do not attach addresses from tombstone index to Select
result. Return error from Get method if address is presented in tombstone
index.

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

pkg/local_object_storage/metabase/delete.go

@@ -0,0 +1,31 @@
+package meta
+
+import (
+	"github.com/nspcc-dev/neofs-api-go/pkg/object"
+	"github.com/pkg/errors"
+	"go.etcd.io/bbolt"
+)
+
+var tombstoneBucket = []byte("tombstones")
+
+// Delete marks object as deleted.
+func (db *DB) Delete(addr *object.Address) error {
+	return db.boltDB.Update(func(tx *bbolt.Tx) error {
+		bucket, err := tx.CreateBucketIfNotExists(tombstoneBucket)
+		if err != nil {
+			return errors.Wrapf(err, "(%T) could not create tombstone bucket", db)
+		}
+
+		if err := bucket.Put(addressKey(addr), nil); err != nil {
+			return errors.Wrapf(err, "(%T) could not put to tombstone bucket", db)
+		}
+
+		return nil
+	})
+}
+
+func objectRemoved(tx *bbolt.Tx, addr []byte) bool {
+	tombstoneBucket := tx.Bucket(tombstoneBucket)
+
+	return tombstoneBucket != nil && tombstoneBucket.Get(addr) != nil
+}