synchronized Set<StoreKey> getMissingStoreKeys() {
      return missingStoreMessages == null ? Collections.emptySet()
          : missingStoreMessages.stream().map(MessageInfo::getStoreKey).collect(Collectors.toSet());
    }
Get the keys corresponding to the missing messages in the local store
@return store keys missing in the local store
ambry-replication/src/main/java/com/github/ambry/replication/ReplicaThread.java