Part #2: update the testcase as the database we're using
was changed.

Make Pushdown_select write output rows into select->join->result, instead of
thd->protocol.

This makes
- SELECT ... INTO @var
- SELECT ... INTO OUTFILE
- INSERT INTO myisam_table SELECT ... FROM clustrix_table
work as intended.

Also fixed the federatedx select pushdown handler:
- Do not fail an assert if the backend no resultset. Produce an error.
- For the SELECT .. INTO syntax, refuse to use Select Handler, because
the impelementation doesn't support this.

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().