Merge volatile module, store persistent and volatile module in root dir

[JiayuZzz]

What is changed and how it works?

Cherry-pick volatile branch, store persistent and volatile module to independent directories.

Check List

Tests

• Unit test
• Stress test

Side effects

No

[iyupeng]

Just 2 questions:

• Will we use different namespaces for persistent and volatile, in the future? Otherwise, these 2 modules may not be compiled into one app.

• Also, will there be some common parts being extracted from both modules, e.g., header files?

[JiayuZzz]

Just 2 questions:

• Will we use different namespaces for persistent and volatile, in the future? Otherwise, these 2 modules may not be compiled into one app.
• Also, will there be some common parts being extracted from both modules, e.g., header files?

1. Yes, this is a todo task. Also, as c API can't be isolated by namespace, we may need to do some rename work.
2. Yes, like Status and common types.