write_mongodb: Adjust to mongo-c-driver v 0.8.1
Add support for:
* Configurable db name
* Capped collection (max size and max doc)
* Single collection (instead of one per plugin)

Configuration example:
<Plugin write_mongodb>
  <Node "local">
    Host "localhost"
    Port "27017"
    Timeout 1000
    StoreRates true
    #Database "mydb"
    #User "auth_user"
    #Password "auth_passwd"

    DbName "mydb" # Change Db name (default: collectd)
    CollectionName "collectd" # Define the unique collection to insert document into (default: plugin name)
    MaxSize 100000  # Create a capped collection of max size
    MaxDoc 50       # (optional) Capped collection can contain max document - MaxSize required or this is ignored

  </Node>
</Plugin>