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.

pkg/policy/id.go

@@ -0,0 +1,64 @@
+/*
+ * Minio Cloud Storage, (C) 2018 Minio, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package policy
+
+import (
+	"encoding/json"
+	"fmt"
+	"regexp"
+)
+
+var idRegexp = regexp.MustCompile("^[[:alnum:]]+$")
+
+// ID - policy ID.
+type ID string
+
+// IsValid - checks if ID is valid or not.
+func (id ID) IsValid() bool {
+	// Allow empty string as ID.
+	if string(id) == "" {
+		return true
+	}
+
+	return idRegexp.MatchString(string(id))
+}
+
+// MarshalJSON - encodes ID to JSON data.
+func (id ID) MarshalJSON() ([]byte, error) {
+	if !id.IsValid() {
+		return nil, fmt.Errorf("invalid ID %v", id)
+	}
+
+	return json.Marshal(string(id))
+}
+
+// UnmarshalJSON - decodes JSON data to ID.
+func (id *ID) UnmarshalJSON(data []byte) error {
+	var s string
+	if err := json.Unmarshal(data, &s); err != nil {
+		return err
+	}
+
+	i := ID(s)
+	if !i.IsValid() {
+		return fmt.Errorf("invalid ID %v", s)
+	}
+
+	*id = i
+
+	return nil
+}