In order to add common code to the value deleter of LRU cache, let all lru cache values inherit from LRUCacheValueBase class and tracking memory in destructor.
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.
