fix: discarding results do not attempt in-memory metacache writer (#11163)

Optimizations include

- do not write the metacache block if the size of the
  block is '0' and it is the first block - where listing
  is attempted for a transient prefix, this helps to
  avoid creating lots of empty metacache entries for
  `minioMetaBucket`

- avoid the entire initialization sequence of cacheCh
  , metacacheBlockWriter if we are simply going to skip
  them when discardResults is set to true.

- No need to hold write locks while writing metacache
  blocks - each block is unique, per bucket, per prefix
  and also is written by a single node.

cmd/erasure-object.go

@@ -674,11 +674,13 @@ func (er erasureObjects) putObject(ctx context.Context, bucket string, object st
 		return ObjectInfo{}, IncompleteBody{Bucket: bucket, Object: object}
 	}
 
-	lk := er.NewNSLock(bucket, object)
-	if err := lk.GetLock(ctx, globalOperationTimeout); err != nil {
-		return ObjectInfo{}, err
+	if !opts.NoLock {
+		lk := er.NewNSLock(bucket, object)
+		if err := lk.GetLock(ctx, globalOperationTimeout); err != nil {
+			return ObjectInfo{}, err
+		}
+		defer lk.Unlock()
 	}
-	defer lk.Unlock()
 
 	for i, w := range writers {
 		if w == nil {