HPCC-8833 Assume keyed join is sorted unless UNORDERED

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
This was there from the first pass at implementing it.  I could remove it now.  I left it in for the moment because it controls whether or not the code generator thinks the join output is sorted.  The engines are unaffected.
I could remove the option if you prefer.