Generalize optimizer to support many frameworks

[jsjason]

The current optimizer is strongly coupled to Dolphin: OptimizationOrchestrator, MetricsReceiver, CostCalculator, and DefaultPlanExecutor. However the optimizer should be as general as possible since our ultimate goal is to support all kinds of frameworks, not only Dolphin. In particular, we should consider how a new asynchronous application framework (dolphin-async?) uses EM and the optimizer.