Sort scheduler debugging dumps in a well-defined order

(From Scott Snyder)

In several places, AvalancheSchedulerSvc dumps out sets of DataObjID objects by iterating over an unordered_set of them. However, the ordering of this iteration is undefined and can, and does, vary between different compilers and different versions of the same compiler. This means that the ordering of the output can vary depending on the compiler, which is inconvenient for regression tests.

This change adds a sorting step so that the DatObjID sets are printed in a well-defined order. This only affects the dumps, so should not be a performance issue.