Persist in-memory replication stats to disk (#15594)

to avoid relying on scanner-calculated replication metrics.
This will improve the accuracy of the replication stats reported.

This PR also adds on to #15556 by handing replication
traffic that could not be queued by available workers to the 
MRF queue so that entries in `PENDING` status are healed faster.

cmd/bucket-replication-utils.go

@@ -766,3 +766,14 @@ func (ri ReplicateObjectInfo) ToMRFEntry() MRFReplicateEntry {
 		versionID: ri.VersionID,
 	}
 }
+
+func getReplicationStatsPath(nodeName string) string {
+	return bucketMetaPrefix + SlashSeparator + replicationDir + SlashSeparator + nodeName + ".stats"
+}
+
+const (
+	replStatsMetaFormat   = 1
+	replStatsVersionV1    = 1
+	replStatsVersion      = replStatsVersionV1
+	replStatsSaveInterval = time.Minute * 5
+)