ReplicaMap#map should better use ConcurrentHashMap rather than HashMap

Even though we use BP write lock in ReplicaMap#initBlockPool method to initial  <bpid,lightWeightResizableGSet> entry,  we'd better change ReplicaMap#map from HashMap to ConcurrentHashMap. Because in ReplicaMap#add or ReplicaMap#addAndGet methods, we only use BP read lock to protect initialization.