kvserver: treat priority update as a success with onEnqueueResult

Previously, bq.addInternal invoked processCallback.onEnqueueResult with a newly
constructed error whenever a replica was already present and re-enqueued with a
higher priority, since the priority had to be passed dynamically. This commit
instead treats the case as a success and passes the updated heap index to
onEnqueueResult, as the caller mainly cares about whether the replica is already
in the queue and its new position.

Author: wenyihu6

pkg/kv/kvserver/queue.go

@@ -894,10 +894,9 @@ func (bq *baseQueue) addInternal(
 			if log.V(1) {
 				log.Infof(ctx, "updating priority: %0.3f -> %0.3f", item.priority, priority)
 			}
-			// TODO(wenyihu6): will this introduce a lot of new memory allocation?
-			processCallback.onEnqueueResult(-1, /*indexOnHeap*/
-				errors.Wrapf(errReplicaAlreadyInQueue, "priority=%.3f->%.3f", item.priority, priority))
 			bq.mu.priorityQ.update(item, priority)
+			// item.index should be updated now based on heap property now.
+			processCallback.onEnqueueResult(item.index /*indexOnHeap*/, nil)
 		}
 		return false, nil
 	}