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/object-api-interface.go

@@ -51,6 +51,7 @@ type ObjectOptions struct {
 	DeleteMarkerReplicationStatus string                 // Is only set in DELETE operations
 	VersionPurgeStatus            VersionPurgeStatusType // Is only set in DELETE operations for delete marker version to be permanently deleted.
 	TransitionStatus              string                 // status of the transition
+	NoLock                        bool                   // indicates to lower layers if the caller is expecting to hold locks.
 }
 
 // BucketOptions represents bucket options for ObjectLayer bucket operations