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/metrics-v3-replication.go

@@ -69,11 +69,12 @@ var (
 // loadClusterReplicationMetrics - `MetricsLoaderFn` for cluster replication metrics
 // such as transfer rate and objects queued.
 func loadClusterReplicationMetrics(ctx context.Context, m MetricValues, c *metricsCache) error {
-	if globalReplicationStats == nil {
+	st := globalReplicationStats.Load()
+	if st == nil {
 		return nil
 	}
 
-	qs := globalReplicationStats.getNodeQueueStatsSummary()
+	qs := st.getNodeQueueStatsSummary()
 
 	qt := qs.QStats
 	m.Set(replicationAverageQueuedBytes, float64(qt.Avg.Bytes))