Re-insert later hot data to memory

[lanlou1554]

When some small data gets evicted from mem_cache, then it becomes hot (frequent access). But in the current implementation, there is no chance for this data to go back to the memory cache.
(Maybe we when get_data_from_cache, we can identify this kind of data, delete it from disk cache, and re-add it into memory cache. But how to identify it?)