add cmd/dump - a dumper

Walk needed to be added to NeedleMap and CompactMap, to be able to add WalkKeys and WalkValues to volume. This is needed for iterating through all the stored needles in a volume - this was dump's purpose.

weed-fs/src/pkg/operation/delete_content.go

@@ -1,8 +1,8 @@
 package operation
 
 import (
-	"net/http"
 	"log"
+	"net/http"
 )
 
 func Delete(url string) error {