Revert storing of TT when returning from "stand pat"

After testing it seems patch is bad:

After 999 games 1+0: +242 -271 =486 -10 ELO

So restore saving of TT at the end but using new Joona
idea of storing as VALUE_TYPE_UPPER/VALUE_TYPE_LOWER instead
of VALUE_TYPE_EXACT.

Some optimization is still possible but better test new ideas
one by one.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>