Fix a possible overflow in TT resize

Fix a possible overflow in TT resize

On platforms where size_t is 32 bit, we
can have an overflow in this expression:

(mbSize * 1024 * 1024)

Fix it setting max hash size of 2GB on platforms
where size_t is 32 bit.

A small rename while there: now struct Cluster
is definied inside class TranspositionTable so
we should drop the redundant TT prefix.

No functional change.