BUG: maybe_convert_objects mixed datetimes and timedeltas 
- [X] corresponding [discussion](https://github.com/pandas-dev/pandas/issues/27417#issuecomment-512014358) 27417
- [X] tests added / passed
- [X] passes `black pandas`
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

In order to stick to `maybe_convert_objects` realization it was needed to introduce `Seen().nat_` param. Since we want to distinguish following situations: were seen `NaT` or `datetime` or `timedelta`

NOTE: in current realization array full of `NaT` will be converted to `np.datetime64`.