Allows guided and adaptive iterators to parallelize when amount of work is less than the number of tasks, instead of simply serializing. See 21813 for additional details. Also moves serialization warnings inside debug conditional.

git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@21822 3a8e244f-b0f2-452b-bcba-4c88e055c3ca