Service for Task Management

[bchocho]

Dolphin requires Task (Group) Management. In particular, triggering optimization relies on receiving metrics from all Tasks that run in that iteration. Keeping track of the number of Tasks running (and thus number metrics that must be received) is done today within the OptimizationOrchestrator. It should be made into a general service.

This is different from #158, which proposes Evaluator management.

[gyeongin]

APIs for tracking and retrieving RunningTask objects will be helpful, since we need this to close the task on purpose. BTW, I think it should be #153, not #158 :)