add ability to wait on subscriptions without an executor

[wjwwood]

The idea is that you can create a subscription, wait on it (and only it) to have a message ready, and then either take the message or have the callback called with that message. This should be possible even if other subscriptions, timers, etc. which are part of the same node are used with an executor. So there should be a way to not add specific subscriptions to an executor or to have the executor ignore it.

Things to consider:

• should it be possible to have a callback group with two subscriptions in it, where one subscription is manually awaited and executed but the other is handled by the executor? (with the executor still respecting the callback group's constraints?)
• probably should do the same thing for timers and service clients/servers at the same time (or at least document how it would be done)

Requires #519