Define the scenario interface

[josephburnett]

Currently various traffic patterns are defined as a golang object that the UI and core simulator both know about. And the same for the metrics returned to the UI. This should be defined as a protobuf similar to #58 so that the UI can be more decoupled from the core. This will allow command line / library usage of Skenario core in addition to the UI.

The proto should include an enumerated set of patterns. But also allow for raw data input (e.g. qps over time).

[jchesterpivotal]

Is the idea that traffic patterns will live in a separate process communicating with the simulator core?

[josephburnett]

No, I just want to have clearly defined traffic shape which the simulator will implement. So the UI can be decoupled from the simulator.