1. Add some comments to make the code easier to understand;
2. Make the metric `create_tablet_requests_failed` to be accurate;
3. Some internal methods use naked pointers directly instead of `shared_ptr`;
4. The `using` in `.h` files are contagious when included by other files,
so we should only use it in `.cpp` files;
5. Some formatting changes: such as wrapping lines that are too long
6. Parameters that need to be modified, use pointers instead of references
No functional changes in this patch.
feef077520