Failed to add/update records when using MongoDB backend

When I submit tasks to either direct or load balanced view using either map or apply_sync I see debug logs below:

```
2018-09-18 23:27:05.936 [IPClusterStart] 2018-09-18 23:27:05.935 [IPControllerApp] ERROR | DB Error updating record u'e5925b97-b0906f91a3415ce89c3c2299'
2018-09-18 23:27:05.936 [IPClusterStart] Traceback (most recent call last):
2018-09-18 23:27:05.937 [IPClusterStart] File "/home/ifu/anaconda2/lib/python2.7/site-packages/ipyparallel/controller/hub.py", line 693, in save_queue_result
2018-09-18 23:27:05.937 [IPClusterStart] self.db.update_record(msg_id, result)
2018-09-18 23:27:05.937 [IPClusterStart] File "/home/ifu/anaconda2/lib/python2.7/site-packages/ipyparallel/controller/mongodb.py", line 87, in update_record
2018-09-18 23:27:05.937 [IPClusterStart] rec = self._binary_buffers(rec)
2018-09-18 23:27:05.937 [IPClusterStart] File "/home/ifu/anaconda2/lib/python2.7/site-packages/ipyparallel/controller/mongodb.py", line 68, in _binary_buffers
2018-09-18 23:27:05.937 [IPClusterStart] rec[key] = list(map(Binary, rec[key]))
2018-09-18 23:27:05.937 [IPClusterStart] File "/home/ifu/anaconda2/lib/python2.7/site-packages/bson/binary.py", line 147, in __new__
2018-09-18 23:27:05.937 [IPClusterStart] raise TypeError("data must be an instance of bytes")
2018-09-18 23:27:05.937 [IPClusterStart] TypeError: data must be an instance of bytes
```

Looks like rec['buffers'] cannot be binarized?

I am running on ubuntu 17.04 with anaconda python 2.7.15, ipyparallel 6.2.2 and pymongo 3.7.0.

Start in ipyparallel/controller/mongodb.py, especially _binary_buffers and update_record, then trace how save_queue_result in ipyparallel/controller/hub.py supplies rec['buffers']. Reproduce a map or apply_sync task with the MongoDB backend and confirm that adding or updating records completes without the BSON bytes TypeError.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.