Optimize host reservation puller

Kea provides no way to signal whether or not host reservations have changed in the database. Thus, Stork always fetches all host reservations for a daemon and updates them in its database. In a typical environment, the host reservations do not change very often and there is no need to update them in the Stork database all the time. In addition, the ticket #680 (closed) introduces config reviews for host reservations. Right now, the config reviews are scheduled every time the puller fetched all reservations. We should optimize the hosts puller to check whether there are any changes to the pulled host reservations and update the reservations in the Stork database only when there were some changes. The config reviews should be scheduled only when at least one host reservation has changed.