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/gateway/s3/gateway-s3_test.go

@@ -52,7 +52,7 @@ func TestS3ToObjectError(t *testing.T) {
 		},
 		{
 			inputErr:    errResponse("NoSuchBucketPolicy"),
-			expectedErr: minio.PolicyNotFound{},
+			expectedErr: minio.BucketPolicyNotFound{},
 		},
 		{
 			inputErr:    errResponse("NoSuchBucket"),