Method Checkpoint

For definition of "check point" see IndexWriter comments: "Clarification: Check Points (and commits)".

Writer calls this when it has made a "consistent change" to the index, meaning new files are written to the index and the in-memory SegmentInfos have been modified to point to those files.

This may or may not be a commit (segments_N may or may not have been written).

We simply incref the files referenced by the new SegmentInfos and decref the files we had previously seen (if any).

If this is a commit, we also call the policy to give it a chance to remove other commits. If any commits are removed, we decref their files as well.