PERF: Improve perf of to_datetime with ISO format
Closes #10178
Closes #8154

Using the example data from the issue:

```
In [2]: df = DataFrame({'date_text':["2015-05-18" for i in range(10**6)]})
```

Before:

```
In [3]: %timeit pd.to_datetime(df['date_text'],format="%Y-%m-%d", box=False).values.view('i8')/10**9
1 loops, best of 3: 3.14 s per loop

In [4]: %timeit pd.to_datetime(df['date_text'],infer_datetime_format=True,     box=False).values.view('i8')/10**9
1 loops, best of 3: 253 ms per lo
```

After:

```
In [6]: %timeit pd.to_datetime(df['date_text'],format="%Y-%m-%d", box=False).values.view('i8')/10**9
1 loops, best of 3: 217 ms per loop

In [7]: %timeit pd.to_datetime(df['date_text'],infer_datetime_format=True, box=False).values.view('i8')/10**9
1 loops, best of 3: 243 ms per loop
```
