to_datetime with format string cannot read nanoseconds
xref #8904

In the format string, `%f` means microseconds (https://docs.python.org/2/library/datetime.html#strftime-and-strptime-behavior), but this means the nanoseconds cannot be read in using a `format` string:

```
In [11]: pd.to_datetime("2012-01-01 09:00:00.000000001", format="%Y-%m-%d %H:%M:%S.%f")
-> 
ValueError: unconverted data remains: 001
```
