BUG: DatetimeIndex with time object as key
closes https://github.com/pydata/pandas/issues/8667
on master:

```
>>> from datetime import time
>>> from pandas.index import _SIZE_CUTOFF
>>> n = _SIZE_CUTOFF + 100
>>> idx = pd.date_range('2014-11-26', periods=n, freq='S')
>>> ts = pd.Series(np.random.randn(n), index=idx)
>>> key = time(15, 0)
>>> ts[key]
TypeError: 'datetime.time' object is not iterable
>>> ts.index.get_loc(key)
TypeError: unorderable types: int() > datetime.time()
```

above would work on master branch if `n` was smaller than `_SIZE_CUTOFF`. 
`_SIZE_CUTOFF` is set [here](https://github.com/pydata/pandas/blob/e5fe75e8d40e6b94eabf54882a17ab05341c9929/pandas/index.pyx#L68)
