Initial implement for initial import for GridFS

- Also content refactoring when GridFS is enabled
Initial import with GridFS enabled #47

- TODO: Initial import with GridFS will still to be optimized
- Directly DBObject instead of Map
- Cleanup the logic with GridFS enabled
- New unit test for initial import with GridFS
- Reduce wait time unit test from 6 sec to 2 sec
- Script filter is not used anymore when GridFS enabled