Implement Savepoints and required metadata timeline (#86)

- Introduce avro to save clean metadata with details about the last commit that was retained
- Save rollback metadata in the meta timeline
- Create savepoint metadata and add API to createSavepoint, deleteSavepoint and rollbackToSavepoint
- Savepointed commit should not be rolledback or cleaned or archived
- introduce cli commands to show, create and rollback to savepoints
- Write unit tests to test savepoints and rollbackToSavepoints

hoodie-client/src/main/java/com/uber/hoodie/exception/HoodieSavepointException.java

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2016 Uber Technologies, Inc. (hoodie-dev-group@uber.com)
+ *
+ * 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 com.uber.hoodie.exception;
+
+public class HoodieSavepointException extends HoodieException {
+
+    public HoodieSavepointException(String msg, Throwable e) {
+        super(msg, e);
+    }
+
+    public HoodieSavepointException(String msg) {
+        super(msg);
+    }
+}