nextWriteDelGen of SegmentCommitInfo should be persisted

When opening an IndexWriter, we will set nextWriteDelGen, nextWriteFieldInfosGen, and nextWriteDocValuesGen beyond what we have in the directory to avoid double writes in cases where the previous IW was not closed gracefully or the new IW is opened with a previous commit (not the latest commit). We need to persist these values; otherwise, they will be lost when the current IW is closed.