io: Add Set/GetByteCounts to TBufferFile

Note to store and restore the larger than 1GB byte count use:

  // Copy the content of the const reference.
  auto bytecounts{b.GetByteCounts()};
  ...
  b.SetByteCounts(std::move(bytecounts));

Author: pcanal

io/io/inc/TBufferFile.h

@@ -117,7 +117,8 @@ class TBufferFile : public TBufferIO {
 
    TObject   *ReadObject(const TClass *cl) override;
 
-   ByteCountFinder_t GetByteCounts() const { return fByteCounts; }
+   const ByteCountFinder_t& GetByteCounts() const { return fByteCounts; }
+   void SetByteCounts(ByteCountFinder_t &&byteCounts) { fByteCounts = std::move(byteCounts); }
 
    using TBufferIO::CheckObject;