[SYSTEMDS-2641] Improved slice finding dml algorithm (perf, correctness)
[SYSTEMDS-2641] Improved slice finding dml algorithm (perf, correctness)

This patch fixes the correctness of the slice finding where the
generation of paired candidates mistakenly used upper.tri instead of
upper.tri-values which created invalid slices and violated the expected
monotonicity properties.

Furthermore this patch also includes two major performance improvements,
specifically (1) the pruning before candidate generation (which does not
affect overall pruning effectiveness due to the handling of missing
parents), and (2) an ID transformation to avoid exceeding integer-max
for many columns and to avoid huge sparse intermediates.

These changes improved the runtime on a replicated (more rows and
columns) salary dataset from 799s to 52s (only 1), and 2.6s (1 and 2)
respectively.
apache/incubator-systemml,apache/incubator-systemml,apache/incubator-systemml,apache/incubator-systemml,apache/incubator-systemml,apache/incubator-systemml