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-cli/src/main/java/com/uber/hoodie/cli/HoodiePrompt.java

@@ -28,18 +28,19 @@ public class HoodiePrompt extends DefaultPromptProvider {
 
     @Override
     public String getPrompt() {
-        String tableName = HoodieCLI.tableMetadata.getTableConfig().getTableName();
-        switch (HoodieCLI.state) {
-            case INIT:
-                return "hoodie->";
-            case DATASET:
-                return "hoodie:" + tableName + "->";
-            case SYNC:
-                return "hoodie:" + tableName + " <==> "
-                    + HoodieCLI.syncTableMetadata.getTableConfig().getTableName() + "->";
-        }
-        if (HoodieCLI.tableMetadata != null)
+        if (HoodieCLI.tableMetadata != null) {
+            String tableName = HoodieCLI.tableMetadata.getTableConfig().getTableName();
+            switch (HoodieCLI.state) {
+                case INIT:
+                    return "hoodie->";
+                case DATASET:
+                    return "hoodie:" + tableName + "->";
+                case SYNC:
+                    return "hoodie:" + tableName + " <==> "
+                        + HoodieCLI.syncTableMetadata.getTableConfig().getTableName() + "->";
+            }
             return "hoodie:" + tableName + "->";
+        }
         return "hoodie->";
     }