If the env or gunicorn has some error, there will not be any error in console
or error log file when we set the subprocss stdout to devnull.
So I set it to None it will inhert its parent process stdout.
When gunicorn cause error, it will show error in console.

Main features:

1. Use the ComputingResourceManager to manage all computing workers.
2. Ensure fair worker number between summary directories at first. So every summary directories in cache will be loaded simultaneously.
3. When a summary directory is loaded, it's worker will be released, and other unfinished summary directories can use the released workers to speed up. This way we solved the slow worker problem.

Code changes:

1. Added computing_resource_mgr.py
2. Passed ComputingResourceManager instances instead of workers_count
3. Simplified the _load_single_fine() function a bit.