This is the first in a series of changes to simplify the complexity in

RelationCounterField, disentangling the responsibilities between

classes and reducing chances of deadlocks or other issues.
In this change, the nested classes (InstanceState and

RelationTracker) have been moved out from RelationTrackerField into

the top level of the new module for the field. This just helps keep

things a bit more organized for the next few steps and reduces the

amount of churn that has to be reviewed.

Unit tests pass.