Draft version of a fix for #302. There is no obvious reused variable to use a...

Draft version of a fix for #302. There is no obvious reused variable to use a temporary seed for each thread, so instead each thread has a thread-local variable which is uses to seed the rand_r call in scheduler_enqueue. I need to test this doesn't fail at any point (e.g. if we're not in a pthread region) but I'm not sure if it would be encountered outside of a pthread @nnrw56?