Capped collection + Notification support

Here are some changes we're using on our monitoring engine allowing:
* capped collection (by size and optionally by document)
* notification write to mongo.
* write to single collection
* Select per node dataset and/or notification write

Capped collection is really handy for 2 reasons: 1/ collection size is automatically controlled so no need to cleanup and 2/ you can use tailable cursor on it.

Please note that this is based on mongodb/mongo-c-driver Legacy version 0.8.1

Thanks

this whole block uses totally unsafe functions