Retire SearchStack sstack[] from SplitPoint

Retire SearchStack sstack[] from SplitPoint

Use a local variable instead. To make it work we need to
correctly init next ply search stack at the beginning of the
search because now that ss is allocated on the stack instead
of on the global storage it contains garbage.

As a side effect we can peform a fast search stack
init in id_loop().

With this patch size of SplitPoint goes from 71944 to 136 bytes,
and consequently size of Thread goes from 575568 to 1104 bytes.

Finally the size of ThreadsManager that contains all the thread
info goes from 9209248 to just 17824 bytes !!

No functional change also in faked split.

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