Refactor write path code by abstract base class. Whether to use `StorageEngine` or `CloudStorageEngine` will be determined during compilation instead of runtime `config::cloud_mode` to avoid unexpected null pointer or undefined behavior issues caused by merging code.
Class that depend on `StorageEngine` but are shared by the cloud mode need to have an abstract base class. Common code should be extracted into the base class, while the code that depends on `StorageEngine` should be implemented in a `StorageEngine` mix-in class of the base class.
6da36e1077