Changes folowing review and further testing

+ Add joinHelper_threads option
+ Add granularity, num threads and limit options
+ Correct a comment
+ Change when CThorSpillableRowArray::flush shuffles ptr array,
with behaviour here, it could cause the row array to keep growing
leaving lots of empty space behind it.
+ Fixed a couple of issue if aborting
+ add array appendRows method for efficiency and clarity
+ add a regression test joinmulti.ecl

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>