Redesign the ExecutionService

The current `ExecutionService` is a placeholder service description and is going to be replaced.

MUST-haves for the new API:

* Allow Execution creation, cancelation, pausing and other lifecycle events
* Allow setting and steering a goal per Execution
* Allow client connections to go away when running an Execution
* Allow consuming the outputs from an Execution from any sequence
* Allow propagation arbitrary headers (e.g. authentication headers) from client to downstream services
* Uniquely identify each Execution and the underlying actor