Provide local storage interface instead of kvs_get_local_data

[llamerada-jp]

Problems to resolve:

• kvs_get_local_data is not efficient. The user program cannot seek locally stored data effectively.
• All data are stored in memory and use a lot of memory when dealing with large data. It looks good to provide the interface to implement a custom module to store local data. So developers can implement the module suitable for each situation.