filer: processing all response headers, no pass through to volume server

* filer calculate MD5 etag
* filer handle response headers, instread of pass it to volume servers

weed/server/filer_server.go

@@ -37,7 +37,6 @@ type FilerOption struct {
 	Masters            []string
 	Collection         string
 	DefaultReplication string
-	RedirectOnRead     bool
 	DisableDirListing  bool
 	MaxMB              int
 	DirListingLimit    int