Fix for a potential null deref in DH for EXPLAIN.

both single row/value UPSERT and bulk UPSERT.

sends down UPDATE statement together with current database name.

Add MTR test for UPDATE.

Multi-UPDATE first sends SELECT to collect constrainsts for the records
to be updated. By default this SELECT uses SH that doesn't suit this.
Add if-block to guard against multi-UPDATE in create_SH().

        Renamed select handler control knob.

        Updated test suite.