Milestone: add support for network-wide events

MD-SAL defines a way for applications to receive notifications within the controller itself, as well as mechanisms for network elements to act as MD-SAL instances.

What we lack is a way for network element agents (e.g. SB plugins) to project a network element's ability to produce notifications, for applications to express the desire to receive a subset of those notifications and a delivery mechanism.

Create a model for the application-level API, allowing applications to define topics and associated node/notification type filters.

Create a model for the network-element-level API, allowing network elements to publish their event capabilities and allowing an aggregatator to instruct them to forward the events using the specified topic encapsulation.

Finally implement an even aggregator, which will act as an implementation of the app-level API and use the NE-level API to create a pub/sub system.