Redesign the ExecutionService

[rakyll]

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