optimize startup sequence performance (#19009)

- bucket metadata does not need to look for legacy things
  anymore if b.Created is non-zero

- stagger bucket metadata loads across lots of nodes to
  avoid the current thundering herd problem.

- Remove deadlines for RenameData, RenameFile - these
  calls should not ever be timed out and should wait
  until completion or wait for client timeout. Do not
  choose timeouts for applications during the WRITE phase.

- increase R/W buffer size, increase maxMergeMessages to 30

cmd/bucket-metadata-sys.go

@@ -22,6 +22,7 @@ import (
 	"encoding/xml"
 	"errors"
 	"fmt"
+	"math/rand"
 	"sync"
 	"time"
 
@@ -482,11 +483,11 @@ func (sys *BucketMetadataSys) concurrentLoad(ctx context.Context, buckets []Buck
 	for index := range buckets {
 		index := index
 		g.Go(func() error {
-			_, _ = sys.objAPI.HealBucket(ctx, buckets[index].Name, madmin.HealOpts{
-				// Ensure heal opts for bucket metadata be deep healed all the time.
-				ScanMode: madmin.HealDeepScan,
-				Recreate: true,
-			})
+			// Sleep and stagger to avoid blocked CPU and thundering
+			// herd upon start up sequence.
+			time.Sleep(25*time.Millisecond + time.Duration(rand.Int63n(int64(100*time.Millisecond))))
+
+			_, _ = sys.objAPI.HealBucket(ctx, buckets[index].Name, madmin.HealOpts{Recreate: true})
 			meta, err := loadBucketMetadata(ctx, sys.objAPI, buckets[index].Name)
 			if err != nil {
 				return err