Change the balancing function

Currently the balancing function is a modified version of the standard
deviation (stddev divided by list length), due to historical reasons.

While this works fine for small clusters, for big clusters it makes
the balancing effect too "weak", and in some cases it refuses to
balance correctly some clusters. It also makes the balancing behaviour
dependant on the cluster size, which is a big no-no.

Therefore we revert to the normal version of standard deviation, and
we also rename the function to reflect what it does. The new version
correctly balances some corner cases that the previous version didn't,
and passes the current balancing unittests.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Adeodato Simo <dato@google.com>