Fix 'generation' type to uint8_t

When we store this value in TT we cut this to 9 bits,
so we need a smaller variable otherwise comparisons
like:

   replace->generation() == generation

Are always false if generation is bigger then the maximum
TT storable value.

This fixes a very nasty and difficult to spot bug (2 weeks for
regression hunting).

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