This PR makes changes to how the list of entities in the executable list is created and managed.

A new class is created, named StaticExecutorEntitiesCollector, that owns the executable list and has the responsibility to keep it up to date and use it to fill the memory strategy.

The StaticExecutorEntitiesCollector provides methods for refreshing the internal executable list whenever nodes are added or removed from the executor, or entities are added or removed from a node.
This class is derived from the Waitable class. This means that it can be added to an executor that will take care of executing the aforementioned methods whenever its required.

The StaticExecutorEntitiesCollector will wake up the executor whenever one of the following guard conditions is triggered:

1. Nodes' guard condition
2. Static executor interrupt guard condition

This results in refreshing the executable list whenever:

1. A new node is added/removed to/from the static executor
2. A new entity is added/removed to/from a node that is part of the static executor

@alsora
@dgoel