InstanceState had a number of methods for setting counter field data

in the database, which was then being called by RelationTracker. By

having these split up, each call in RelationTracker had to be

accompanied by additional calls for synchronizing the values to other

model instances in other states, complicating those calls. It also meant

that InstanceState had to own a handful of functions that didn't

really need to be there.
This change moves those out into RelationTracker, and in the process

simplifies each of their implementations by using a contextmanager that

takes care of splitting up the states into "main" and "other" states

(removing the need for _separate_saved_states()) and then handling the

synchronizing (simplifying all call sites).
This is the second in a series of changes for simplifying the

implementation of RelationCounterField and making it less prone to

deadlocking issues.

All unit tests pass.