Fix uninitialized replication stats (#20260)

Services are unfrozen before `initBackgroundReplication` is finished. This means that 
the globalReplicationStats write is racy. Switch to an atomic pointer.

Provide the `ReplicationPool` with the stats, so it doesn't have to be grabbed 
from the atomic pointer on every use.

All other loads and checks are nil, and calls return empty values when stats 
still haven't been initialized.

cmd/bucket-replication-stats.go

@@ -87,6 +87,9 @@ func (r *ReplicationStats) updateMovingAvg() {
 
 // ActiveWorkers returns worker stats
 func (r *ReplicationStats) ActiveWorkers() ActiveWorkerStat {
+	if r == nil {
+		return ActiveWorkerStat{}
+	}
 	r.wlock.RLock()
 	defer r.wlock.RUnlock()
 	w := r.workers.get()
@@ -351,6 +354,9 @@ func NewReplicationStats(ctx context.Context, objectAPI ObjectLayer) *Replicatio
 }
 
 func (r *ReplicationStats) getAllLatest(bucketsUsage map[string]BucketUsageInfo) (bucketsReplicationStats map[string]BucketStats) {
+	if r == nil {
+		return nil
+	}
 	peerBucketStatsList := globalNotificationSys.GetClusterAllBucketStats(GlobalContext)
 	bucketsReplicationStats = make(map[string]BucketStats, len(bucketsUsage))
 
@@ -460,6 +466,9 @@ func (r *ReplicationStats) calculateBucketReplicationStats(bucket string, bucket
 
 // get the most current of in-memory replication stats  and data usage info from crawler.
 func (r *ReplicationStats) getLatestReplicationStats(bucket string) (s BucketStats) {
+	if r == nil {
+		return s
+	}
 	bucketStats := globalNotificationSys.GetClusterBucketStats(GlobalContext, bucket)
 	return r.calculateBucketReplicationStats(bucket, bucketStats)
 }
@@ -495,9 +504,14 @@ func (r *ReplicationStats) decQ(bucket string, sz int64, isDelMarker bool, opTyp
 
 // incProxy increments proxy metrics for proxied calls
 func (r *ReplicationStats) incProxy(bucket string, api replProxyAPI, isErr bool) {
-	r.pCache.inc(bucket, api, isErr)
+	if r != nil {
+		r.pCache.inc(bucket, api, isErr)
+	}
 }
 
 func (r *ReplicationStats) getProxyStats(bucket string) ProxyMetric {
+	if r == nil {
+		return ProxyMetric{}
+	}
 	return r.pCache.getBucketStats(bucket)
 }