Split collector

The collector grew from doing one thing to doing too many. This encumber
scalability and he's really a bad design as it is. Let's fix that.

This patch take out the notification handling code to a new daemon
called ceilometer-agent-notification. This daemon will be in charge of
receiving notifications, storing them if needed, and building samples in
from of it.

Change-Id: I093f3b7855bd6ffff2018db9cd485ed2fc0f37a5
Blueprint: split-collector