revising prepareThreshold and binaryTransfer behavior.

Hi,

I revied prepareThreshold and binaryTransfer behavior.
This change make the driver work the same behavior with 9.2-1004 driver
and if you specify -1 to prepareThreshold, it works to make first 
preparedStatement use binary encoding transfer.
If you repeatedly execute a query, getting Describe is executed on only first time.

regards,
Tomonari Katsumata