This DB follow the design from the LevelDB
This db will be a key-value store. The key will be a string and the value will be a string.
Only support the key-based search, do not provide any index mechanism.
std::string Get(std::string)
bool Put(std::string, str::string)class SkipList { private: Node* head_; const int kMaxHeight; }
Find: FindGreaterOrEqual(std::string_view key, Node** prev)