scheduler: overhaul util.go/tasksUpdated

[shoenig]

tasksUpdated is a critical block of code that determines whether a replacement alloc will be necessary upon diffing the task group of two jobs. However it provides no trace of why a task group is determined to need replacing - it merely returns a boolean and logs nothing.

Also we should stop using reflect.DeepEqual for comparison here (or anywhere). It does not do the "right" thing when comparing e.g. nil vs empty slices, and has other gotchas. We should write Equal methods for every type, or defer to go-cmp with custom comparators.

Context: in investigating #16238 it seems something has broken in-place updates for services but it's not clear what.