collect statistics for first block in ultra mode

collect statistics for first block in ultra mode

this patch makes btultra do 2 passes on the first block,
the first one being dedicated to collecting statistics
so that the 2nd pass is more accurate.

It translates into a very small compression ratio gain :

enwik7, level 20:
blocks  4K : 2.142 -> 2.153
blocks 16K : 2.447 -> 2.457
blocks 64K : 2.716 -> 2.726

On the other hand, the cpu cost is doubled.

The trade off looks bad.
Though, that's ultimately a price to pay to reach better compression ratio.
So it's only enabled when setting btultra.