re-use the same local drive used by remote-peer (#18645)

historically, we have always kept storage-rest-server
and a local storage API separate without much trouble,
since they both can independently operate due to no
special state() between them.

however, over some time, we have added state()
such as

- drive monitoring threads now there will be "2" of
  them per drive instead of just 1.

- concurrent tokens available per drive are now twice
  instead of just single shared, allowing unexpectedly
  high amount of I/O to go through.

- applying serialization by using walkMutexes can now
  be adequately honored for both remote callers and local
  callers.

cmd/bucket-replication.go

@@ -3263,7 +3263,11 @@ func (p *ReplicationPool) persistToDrive(ctx context.Context, v MRFReplicateEntr
 		return r
 	}
 
-	for _, localDrive := range globalLocalDrives {
+	globalLocalDrivesMu.RLock()
+	localDrives := globalLocalDrives
+	globalLocalDrivesMu.RUnlock()
+
+	for _, localDrive := range localDrives {
 		r := newReader()
 		err := localDrive.CreateFile(ctx, minioMetaBucket, pathJoin(replicationMRFDir, globalLocalNodeNameHex+".bin"), -1, r)
 		r.Close()
@@ -3325,7 +3329,12 @@ func (p *ReplicationPool) loadMRF() (mrfRec MRFReplicateEntries, err error) {
 
 		return re, nil
 	}
-	for _, localDrive := range globalLocalDrives {
+
+	globalLocalDrivesMu.RLock()
+	localDrives := globalLocalDrives
+	globalLocalDrivesMu.RUnlock()
+
+	for _, localDrive := range localDrives {
 		rc, err := localDrive.ReadFileStream(p.ctx, minioMetaBucket, pathJoin(replicationMRFDir, globalLocalNodeNameHex+".bin"), 0, -1)
 		if err != nil {
 			continue