Merge pull request !442 from wangshuide/wsd_handle_special_tensor

For max processes used for computing, we need to make sure every summary directory has a process to load data. We also should not use too many processes to avoid system problems (eg. out of memory). So we calc the max processes cnt in _calc_default_max_processes_cnt

For _load_data_in_thread_wrapper, because self._load_data_in_thread() will create process pool when loading files, we can not afford to run multiple self._load_data_in_thread() simultaneously. So we use a lock to make sure that only one self._load_data_in_thread() is running.

1. To accelerate summary file parsing, multiple processes are used. As the first step to mindinsight parsing performance optimization, we only made changes to _load_single_file function.

2. This PR will imporve summary parsing throughput dramatically (about cpu_count times)

3. Changes are mainly about _load_single_file function

In the future, a more global concurrent computing framework is needed for mindinsight. See the gitee wiki doc for details.