Fix DBEnv's set_tx_timestamp wrapper to be slightly more correct on

Fix DBEnv's set_tx_timestamp wrapper to be slightly more correct on
non-32bit platforms. Will still only allow 32 bits in a timestamp on Win64,
but at least it won't crash, and it'll work right on platforms where longs
are big enough to contain time_t's.

(A better-working, although conceptually less-right fix would have been to
use Py_ssize_t here, but Martin and Tim won't let me.)