A number of bugfixes caused by some of the changes in the arithmetic
type-coercion rules, and the parser-framework changes.

COUNT(DISTINCT a) wasn't being handled correctly due to an issue in
the NanoSQLTranslator flow-control.

The SumAvg/StdDevVar aggregate implementations are now updated to
produce double-precision results when calculating their values.  The
type-converter had been changed to do integer division when the LHS
and RHS are both integers, and this was breaking tests of these
aggregate functions.

Somehow when upgrading the NanoDB parser to ANTLR4, I forgot to include
the SHOW STATS command.  This is essential for the storage lab, so of
course I added it back in as quickly as possible.