Restore: Have RESTORECORE set the UpdateLog state

This is a refactoring improvement designed to simplify & clarify a step in collection restores. One of the final phases of RestoreCmd (collection restore) is to call REQUESTAPPLYUPDATES on each newly restored replica in order to transition the state of the UpdateLog to ACTIVE (not actually to apply updates). The underlying call on the UpdateLog could instead be done inside RESTORECORE at the end with explanatory comments as to the intent. I think this makes more sense that RESTORECORE finish with its UpdateLog ready. And it's strange/curious to see requests in the cluster to apply updates from an updateLog when there is none to do! Adding clarifying comments is important.

See my comment: https://issues.apache.org/jira/browse/SOLR-12065?focusedCommentId=17751792&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-17751792

I think there isn't any back-compat concern.