tighten sync block on mersennetwister init

could be made tighter yet, or even remove the sync with more work
setup to run large covtype as a junit

use ENTROPY instead of GINI, more depth, etc.
Optimize the Entropy.ltSplit method some

Keep running distribution totals, instead of recomputing each loop.
Keep track of distributions with but a single class (these have zero
gain).
Do not break ties randomly; nearly all ties are due to zero-gain splits
in leaves - which is time-expensive (see comment on *billions* of
tie-breaks) for miniscule prediction gain.