1. Ease the elegant exiting procedure by providing a singleton access pattern. Now the exiting procedure can easily call the exit_non_block method to notify the executors to start exiting worker processes. If the worker processes not exit after a timeout, the mindinsight stop still can kill these processes.

2. Build a framework for managing computing resources used by different businesses. Different businesses can now call the get_executor() API to get their process pool to execute compute heavy tasks outside the mindinsight guinicorn worker process, which will make the restful api respond more quickly.

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.