kv: allocator should apply all span config changes proactivily about cockroach

[arulajmani]

When a span config change is made it should apply immediately. In some cases the cluster waits until the system has scanned through to find the range instead of applying it immediately.

We spoke about this in one of our meetings, so I'll capture the discussion here as well for others. The expectation is that this should already happen. See:

cockroach/pkg/kv/kvserver/store.go

Lines 2550 to 2552 in 3b969e3

	// onSpanConfigUpdate is the callback invoked whenever this store learns of a
	// span config update.
	func (s *Store) onSpanConfigUpdate(ctx context.Context, updated roachpb.Span) {

@andrewbaptist mentioned that anecdotally, he's seen rare cases when this doesn't happen. We should clarify these.