The recent fix for tracking multiple instances in RelationCounterField

introduced a deadlock that manifested during a Review Board test suite

run. It was possible for a weak reference to drop while adding a new

instance, causing _reset_state to grab a locl while _store_state was

running (as part of the instantiation of an InstanceState).
We now delay setting up InstanceState until we leave the lock,

allowing the lock to just center around manipulating the state map, and

allowing setup/destroying to be in its own locks.

My reliably-broken unit test run no longer deadlocks.
All Djblets and Review Board unit tests pass.