fix prepareThreshold bug

A commit(dbf76c2d66) was committed for the people who want to
use binaryTransfer at first time preparedStatement execution.

The change was good for the aim, but the compatibility with
pre-9.2 driver has been broken and the performance has been
degraded.

I fixed this problem.
My solution is to introduce preparedThreshold=-1 for forcing
binaryTransfer.

I want to fix master and 93_STABLE.

regards,

Tomonari Katsumata