fix Rolling for multi-index and reversed index.
Fix Rolling operation for level of multi-index and descending time index (that is monotonic, but decreasing).

- [x] closes #19248
- [x] closes #15584
- [x] tests passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Does this close a particular issue? Also tests are the most critical part of any PR so would be great if you can add > Does this close a particular issue? 

I found those two just now:
https://github.com/pandas-dev/pandas/issues/19248
https://github.com/pandas-dev/pandas/issues/15584

> Also tests are the most critical part of any PR so would be great if you can add

Will do.

 Hello @leftys! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:





There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2019-10-19 20:11:05 UTC @WillAyd I added tests and updated whatsnew to reference the two issues. can you update the top section with the issue references (use a closes #....) @jreback Fixed according to review comments. Thanks @leftys - very nice PR!