Enhance policy handling to support SSE and WORM (#5790)

- remove old bucket policy handling
- add new policy handling
- add new policy handling unit tests

This patch brings support to bucket policy to have more control not
limiting to anonymous.  Bucket owner controls to allow/deny any rest
API.

For example server side encryption can be controlled by allowing
PUT/GET objects with encryptions including bucket owner.

cmd/admin-handlers_test.go

@@ -181,6 +181,9 @@ func prepareAdminXLTestBed() (*adminXLTestBed, error) {
 		return nil, err
 	}
 
+	// Create new policy system.
+	globalPolicySys = NewPolicySys()
+
 	// Setup admin mgmt REST API handlers.
 	adminRouter := mux.NewRouter()
 	registerAdminRouter(adminRouter)