code: void register(CloudantService svc, ConcurrentMap<ClientKey, Object> clients) {
          registrations.put(svc, clients);
      }
docs: Lazily registers a CloudantService to have its client cache purged of entries related to a stopped application.
  
  @param svc CloudantService instance. This is used as a HashMap key because the client cache itself is not suitable as a key.
  @param clients client cache for the CloudantService.