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/data-crawler.go

@@ -570,8 +570,13 @@ func (f *folderScanner) scanQueuedLevels(ctx context.Context, folders []cachedFo
 							bucket:    bucket,
 							object:    entry.name,
 							versionID: "",
+							opts: &madmin.HealOpts{
+								Remove: true,
+							},
 						}, madmin.HealItemObject)
-						logger.LogIf(ctx, err)
+						if !isErrObjectNotFound(err) && !isErrVersionNotFound(err) {
+							logger.LogIf(ctx, err)
+						}
 						foundObjs = foundObjs || err == nil
 						return
 					}
@@ -583,8 +588,13 @@ func (f *folderScanner) scanQueuedLevels(ctx context.Context, folders []cachedFo
 							bucket:    bucket,
 							object:    fiv.Name,
 							versionID: ver.VersionID,
+							opts: &madmin.HealOpts{
+								Remove: true,
+							},
 						}, madmin.HealItemObject)
-						logger.LogIf(ctx, err)
+						if !isErrObjectNotFound(err) && !isErrVersionNotFound(err) {
+							logger.LogIf(ctx, err)
+						}
 						foundObjs = foundObjs || err == nil
 					}
 				},