[#922] storage engine: Prevent any operations after first Close call

Make `BlockExecution` / `ResumeExecution` to not release per-shard worker
pools. Make `StorageEngine.Close` to block these methods and any
data-related operations. It is still releases the pools.

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

pkg/local_object_storage/engine/engine_test.go

@@ -116,3 +116,13 @@ func addAttribute(obj *object.RawObject, key, val string) {
 	attrs = append(attrs, attr)
 	obj.SetAttributes(attrs...)
 }
+
+func testNewEngineWithShardNum(t *testing.T, num int) *StorageEngine {
+	shards := make([]*shard.Shard, 0, num)
+
+	for i := 0; i < num; i++ {
+		shards = append(shards, testNewShard(t, i))
+	}
+
+	return testNewEngineWithShards(shards...)
+}