fix: inherit heal opts globally, including bitrot settings (#11166)

Bonus re-use ReadFileStream internal io.Copy buffers, fixes
lots of chatty allocations when reading metacache readers
with many sustained concurrent listing operations

```
   17.30GB  1.27% 84.80%    35.26GB  2.58%  io.copyBuffer
```

cmd/storage-rest-server.go

@@ -522,7 +522,10 @@ func (s *storageRESTServer) ReadFileStreamHandler(w http.ResponseWriter, r *http
 
 	w.Header().Set(xhttp.ContentLength, strconv.Itoa(length))
 
-	io.Copy(w, rc)
+	bufp := s.storage.pool.Get().(*[]byte)
+	defer s.storage.pool.Put(bufp)
+
+	io.CopyBuffer(w, rc, *bufp)
 	w.(http.Flusher).Flush()
 }