Refresh TT entry after a cut-off to avoid aging

Re-save the same TT entry if value is usable and allow
us to cut-off, it means that entry is valuable and
we want to keep it fresh updating the 'generation'
parameter up to the current value.

Patch suggested by J. Wesley Cleveland and better
clarified by Miguel A. Ballicora.

After 999 games at 1+0 64MB hash size
Mod vs Orig +167 =677 -155 +4 ELO

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