BUG: GH4017, efficiently support non-unique indicies with iloc

closes #4017

This was a bug because the iloc was dealing with a non-unique index (and was
reindexing which is not correct in this situation, instead can effectively
take)

```
In [1]: df= DataFrame({'A' : [0.1] * 300000, 'B' : [1] * 300000})

In [2]: idx = np.array(range(3000)) * 99

In [3]: expected = df.iloc[idx]

In [4]: df2 = DataFrame({'A' : [0.1] * 100000, 'B' : [1] * 100000})

In [5]: df2 = pd.concat([df2, 2*df2, 3*df2])

In [6]: %timeit df2.iloc[idx]
1000 loops, best of 3: 221 us per loop

In [7]: %timeit df2.loc[idx]
10 loops, best of 3: 25.6 ms per loop

```