re-implement data usage crawler to be more efficient (#9075)

Implementation overview: 

https://gist.github.com/klauspost/1801c858d5e0df391114436fdad6987b

cmd/storage-interface.go

@@ -17,6 +17,7 @@
 package cmd
 
 import (
+	"context"
 	"io"
 )
 
@@ -32,7 +33,7 @@ type StorageAPI interface {
 	SetDiskID(id string)
 
 	DiskInfo() (info DiskInfo, err error)
-	CrawlAndGetDataUsage(endCh <-chan struct{}) (DataUsageInfo, error)
+	CrawlAndGetDataUsage(ctx context.Context, cache dataUsageCache) (dataUsageCache, error)
 
 	// Volume operations.
 	MakeVol(volume string) (err error)