This patch does not implement support for partial keys, it just adds the
parameter for the Clustrix command.

This patch also fixes a bug in scan_end() and removes an unneeded call
to set_overwrite_status().

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().