Method OnCommit

this is called each time the writer completed a commit. this gives the policy a chance to remove old commit points with each commit.

The policy may now choose to delete old commit points by calling method Delete() of IndexCommit.

This method is only called when Commit()} or Dispose() is called, or possibly not at all if the Rollback()} method is called.

Note: the last CommitPoint is the most recent one, i.e. the "front index state". Be careful not to delete it, unless you know for sure what you are doing, and unless you can afford to lose the index content while doing that.