BUG/CLN: clarified timedelta inferences

closes #5458
closes #5695
closes #5689

cleaned up / clarified timedelta inferences, bottom line is the following works, previously these would
return not-useful object dtypes

```

In [4]: Series([np.timedelta64(1,'s'),timedelta(days=1),pd.NaT])
Out[4]: 
0   0 days, 00:00:01
1   1 days, 00:00:00
2                NaT
dtype: timedelta64[ns]

```