Merge PR #17657 into master

Merge PR #17657 into master

* refs/remotes/upstream/pull/17657/head:
	mds: optimize MDCache::rejoin_scour_survivor_replicas()
	mds: fix MDSCacheObject::clear_replica_map
	mds: support limiting cache by memory
	common: refactor of lru
	mds: resolve unsigned coercion compiler warning
	common: use safer uint64_t for list size
	common: add bytes2str pretty print function
	mds: check if waiting is allocated before use
	mds: go back to compact_map for replicas
	mds: use mempool for cache objects
	mds: cleanup replica_map access
	common: add alloc_ptr smart pointer
	common: add warning on base class use of mempool
	common: use atomic uin64_t for counter

Reviewed-by: Zheng Yan <zyan@redhat.com>