RelationCounterField keeps state around in weak references, and

attempts to delete entries from that state when instances go away. On

occasion, we'd see a crash from within weakref about a dictionary

being modified during iteration, and this happened while we were simply

gathering the keys and values into a list.
Most likely, this was happening due to two threads altering the states

at the same time. In order to prevent this, we now have a _state_lock

that's used whenever something wants to modify the state dictionaries,

allowing only one to make a modification at a time.
There's also a change to how state is reset. Instead of copying the

entire set of keys and values out of the dictionary and then changing

the dictionary inside the iteration, we now iterate without copying and

store the keys to delete in a list. We then separately iterate through

those and delete from the dictionary, saving the amount of data that

must be copied around.

Unit tests pass.
This is a really difficult thing to write a test for, so we're going to

settle for seeing whether we continue to get errors in production.