Create common structures for executors to use

[mjcarroll]

Broken off of #2142 so that it can be used as a base for #2140.

This introduces a few main structures:

• ExecutorNotifyWaitable - an object used to collect node and callback group guard conditions from things associated with an executor.
• ExecutorEntitiesCollector - an object to collect entities from executor-associated nodes and callback groups
• ExecutorEntitiesCollection - a collection of entities that are associated with an executor via callback groups and nodes.

CC: @alsora @mauropasse