option to continously update zts domain cache files

[havetisyan]

To keep the cached domain files in ZTS up-to-date, the server needs to periodically fetch the domain files and update the local copy. This is also necessary when ZMS server rotates its private key, and we need to make sure all cached files are updated before retiring the old private key.

Now, the server will periodically refresh the "stale" domain files based on the refresh timeout configuration setting. In order not to generate too much load on the ZMS server, we're not going to refresh all domain files at once but during each run we'll only process the configured number of domains.