Add support for session variables and SELECT..INTO into Select Handler

As of 5.4.1 MCS quits early from create_columnstore_select_handler() if SELECT..INTO is detected disabling SH for the queries like SELECT c1 INTO @var FROM t or SELECT * INTO OUTFILE '/tmp/some.data' FROM t.
We need to re-evaluate the block and test MCS with the queries that contains SELECT..INTO.

    if (((thd->lex)->result &&

        !((select_dumpvar *)(thd->lex)->result)->var_list.is_empty()) &&

        (!isPS))

    {

        return handler;

    }