fix: metacache should only rename entries during cleanup (#11503)

To avoid large delays in metacache cleanup, use rename
instead of recursive delete calls, renames are cheaper
move the content to minioMetaTmpBucket and then cleanup
this folder once in 24hrs instead.

If the new cache can replace an existing one, we should
let it replace since that is currently being saved anyways,
this avoids pile up of 1000's of metacache entires for
same listing calls that are not necessary to be stored
on disk.

cmd/metacache-manager.go

@@ -92,7 +92,6 @@ func (m *metacacheManager) initManager() {
 			}
 			m.mu.Unlock()
 		}
-		m.getTransient().deleteAll()
 	}()
 }
 
@@ -124,11 +123,11 @@ func (m *metacacheManager) updateCacheEntry(update metacache) (metacache, error)
 	}
 
 	b, ok := m.buckets[update.bucket]
+	m.mu.RUnlock()
 	if ok {
-		m.mu.RUnlock()
 		return b.updateCacheEntry(update)
 	}
-	m.mu.RUnlock()
+
 	// We should have either a trashed bucket or this
 	return metacache{}, errVolumeNotFound
 }