SERVER-16621 emit at mapper may run out of memory

The mapper may generate lots of items and each item is kept in-memory at this state. It would be crashed when run out of memory.

NOTE: Skip the checking when using v8 engine (_jsMode: true). The best way is checking at emit function and transfers in memory storage to temp collection. Current state is checking at each document handled because the framework flow designed.
===
https://github.com/changyy/mongo-mapper-emit-out-of-memory/

Job is done via jsMode=true.
Mongod crashed via jsMode = false:
https://github.com/changyy/mongo-mapper-emit-out-of-memory/blob/master/index.js#L91
```
$ tail -f /var/log/mongodb/mongod.log
...
Assertion: 10000:out of memory BufBuilder
```

The purpose of mapper: 
```
INPUT: ["1", "2", "3"]
OUTPUT: ["1-1", "1-2", "2-1", "1-3", "3-1", "2-2", "2-3", "3-2", "3-3"]
```

Execute the index.js and restart the mongod. (it crashed because out of memory).

No item_pair_jsmode_off (jsMode = false):
```
> show collections
item_pair_jsmode_on
rec
system.indexes
> db.item_pair_jsmode_on.count()
4163817
```