This fixes a variant of SummingMergeTree() in which
the columns to sum are explicitly configured.
Previously columns not in that list were ignored,
instead of writing last value.
This also fixes summation of invalid maps with
with only one key column and no value columns.
Modified test to work around compaction limitation
in which a zero-value column isn’t compacted
immediately if the inputs are non-zero but the
output is zero (+1 -1).
