If
DirectoryReader#open(IndexWriter,boolean) has been called (ie, this writer is in near real-time mode), then after a merge completes, this class can be invoked to warm the reader on the newly merged segment, before the merge commits. this is not required for near real-time search, but will reduce search latency on opening a new near real-time reader after a merge completes. @lucene.experimental
NOTE: warm is called before any deletes have been carried over to the merged segment.
