Add online Lucene reindexing support

At startup time, if the most recent write version is not ready for
searching, kick off a background thread to do a batch reindex. When
that finishes, swap the new search version into the IndexCollection.

Solr continues to use a single version determined at startup for now.

Change-Id: I99c53b7df1c0bec401aac8184071e04e75e8a972
5 files changed