Backup_context is copied several times during backup process

The Backup_context object should been one copy during backup process, and the all function pass it by reference. However, in function backup_files bind the argument by value, that will cause needless copy, destructor. And, if the Backup_context contain some global status, it may cause unexpected error.

Improvement method:

 bool backup_files(const char *from, bool prep_mode, Backup_context &context) {

   run_data_threads(from,
                    std::bind(backup_thread_func, std::placeholders::_1,

•                             prep_mode, rsync_tmpfile, context),
  +                             prep_mode, rsync_tmpfile, std::ref(context)),
                      xtrabackup_parallel, "backup");