Feature Reqeust: shard LRU list

[zhangjinpeng87]

AgateDB keep all files open, both SST files and VLog files. If the dataset is large and there are many files, most files may not will touch in a long time. In this scenario we may need a LRU list to maintain those most active files to keep the number of opened files under a specified limit.

Also, after we introduced block cache which used to keep the max memory usage of AgateDB under a limitation to prevent OOM issue, we need the LRU list to maintain those active data blocks.