Metacache add abs entry limit (#11483)

Add an absolute limit to the number of metacaches for a bucket.

Delete excess caches if they haven't been handed out in an hour.

cmd/metacache-manager.go

@@ -43,6 +43,7 @@ type metacacheManager struct {
 }
 
 const metacacheManagerTransientBucket = "**transient**"
+const metacacheMaxEntries = 5000
 
 // initManager will start async saving the cache.
 func (m *metacacheManager) initManager() {