Javadoc
Tries to locate and execute tasks for a stealer of the given
task, or in turn one of its stealers, Traces
currentSteal->currentJoin links looking for a thread working on
a descendant of the given task and with a non-empty queue to
steal back and execute tasks from. The implementation is very
branchy to cope with potential inconsistencies or loops
encountering chains that are stale, unknown, or of length
greater than MAX_HELP links. All of these cases are dealt with
by just retrying by caller.