Deterministic Configurable Registration Options

[bennowotny]

Problem

• wready servers schedule clients indeterministically (whenever the client tells it to), causing operator confusion when wready is working with HCI components.
• wready has no way of knowing when no other clients will register, causing it to remain active during entire ROS launches, even if it no longer serves any function.

Proposed Solution

• Add an option to wready launches as a configurable list of expected clients. These clients are identified by string keys that the client must use to start a wready transaction with a server that has such an option configured. Any of these elements of the sequence should be user-skippable to avoid a deadlock.
• If wready is configured with this option and all schedule participants have been completed/skipped, then the node will shut itself off.