signature: Use a layered approach for signature verification.

Signature calculation has now moved out from being a package to
top-level as a layered mechanism.

In case of payload calculation with body, go-routines are initiated
to simultaneously write and calculate shasum. Errors are sent
over the writer so that the lower layer removes the temporary files
properly.

generic-handlers.go

@@ -28,11 +28,6 @@ import (
 	"github.com/rs/cors"
 )
 
-const (
-	iso8601Format = "20060102T150405Z"
-	privateBucket = "/minio"
-)
-
 // HandlerFunc - useful to chain different middleware http.Handler
 type HandlerFunc func(http.Handler) http.Handler
 
@@ -51,6 +46,11 @@ type redirectHandler struct {
 	locationPrefix string
 }
 
+// Private bucket.
+const (
+	privateBucket = "/minio"
+)
+
 func setBrowserRedirectHandler(h http.Handler) http.Handler {
 	return redirectHandler{handler: h, locationPrefix: privateBucket}
 }