Feature: add an env to enable or disable block cache of an app

[ZhongChaoqiang]

Sometimes, there are some apps of different business in a cluster.Some apps want to enable block cache, others my be not.
So we need add an env to enable or disable block cache of an app.

We control it by fill_cache option of rocksdb::ReadOptions.
If fill_cache is true, when reading data from rocksdb, the data block will be filled into cache.
If fill_cache change to false,the data block will not be filled into cache.But we do not need to clear the block of cached.When block cache is full, the oldest block will be remove by LRU.