A mapping of instances keyed by their id.
Stores instances by their id. Holds instances based on reference counts which are incremented by addInstanceReference and decremented by deleteInstanceReference. Once a reference count is 0, the instance is no longer held in the store. Once a reference count is greater than 0, the instance is added to the store.
connection.addInstanceReference(todo5); connection.instanceStore.get("5") //-> todo5