DateUtil guessing can interpret data in different formats

[domoritz]

Since the DateUtil type only tries to convert a string into a date, It might interpret different rows with different formats.

Example:

02/03/04 -> MM/DD/YY
31/03/04 -> DD/MM/YY

This is why we have the Date type but it is very slow because it has to go through different formats. A way to speed it up is to reduce the number of possible formats when using the strict type guessing. Only date formats that are still possible should be used for following rows.