refactor(cache): lazy initialize rwlock to reduce memory usage

[xiaoyang-hhh]

Change LruEntry::rw_lock_ from direct member to std::unique_ptr for lazy initialization in Full File Cache:

• rw_lock_ is now created when the openCount in LruEntry changes from 0 to 1, and destroyed when it changes from 1 to 0
• Add spinlock to protect rw_lock_ pointer during init/destroy

This reduces memory usage when there are many cache files, but only a small number of them are being read or written at any moment.

[lihuiba]

given that you bind the life-cycle of rwlock to FileStore, is it possible to move the rwlock from LruEntry to FileStore?

[xiaoyang-hhh]

given that you bind the life-cycle of rwlock to FileStore, is it possible to move the rwlock from LruEntry to FileStore?

The previous implementation had some issues. A cached file may correspond to multiple CacheStores meanwhile, so the rwlock is still kept in LruEntry, with its creation and destruction controlled by openCount.