objects range to be synced include start and end

[chnliyong]

1. Add Head method for ObjectStorage and remove Exists method. The Exists method can be replaced like this:

   err := store.Exists(key)  ==>  _, err := store.Head(key)

2. Modify ListAll method's behavior for sftp and file. To make sftp and file consistent with Object Storage , if the prefix is ends with / , prefix is the start point to traverse, otherwise the filepath.Dir(prefix) would be listed for the case like this:

   temp
   ├── x
   │   └── y
   │       └── z.txt
   ├── xy
   ├── xy.txt
   └── xyz
   └── xyz.txt

   where the prefix is ./temp/x .