Add capability to enable MRC histogram generation on a pool in Ceph

by Peilun for Ceph Storage

MRC is one of the most useful metrics to analyze cache performance. We want to implement a new method SHARDS which makes it possible to generate MRC online for a large system like Ceph. This data can be provided to developers to develop better cache strategies, can can also be provided to users to choose a better cache size. And some other optimizations like better LRU stack management or compression will be applied so that it can have a better performance.