Gnocchi performs better if measurements are write in batch
When Ceilometer is used with Gnocchi, this is not possible.
This change introduce a new notification listener that allows that.
On the driver side, a default batch implementation is provided.
It's just call the legacy poll method many times.
Driver can override it to provide a better implementation.
For example, kafka handles batch natively and take benefit of this.
Change-Id: I16184da24b8661aff7f4fba6196ecf33165f1a77
4dd644ac20