[Flytekit] Centralized Control Plane Class to handle fetching, launching of flyte entities

[cosmicBboy]

See #823 for additional background.

The new control plane classes introduced in #425 naively ports over some of the functionality from the old control plane classes. The thing we want to change is to implement a centralized control plane class (name TBD) that handles the fetching and launching of the different flyte entities (e.g. fetching a workflow and launching it with some parameters). This is instead of having fetch / launch methods attached to each flyte entity class.

• List recent workflow executions for a given project/domain/workflow name.
• Cache flyte entities when doing fetch

[cosmicBboy]

fixed by #547