Remove owner field from TwoPhaseVolatileStore

We associated states in TPVS with the creating (or using) thread. This really doesn't make sense, since locking can be done elsewhere if necessary and it's not a property that is required by the object store implementation or supported by other implementations.

I can't recall why I did this in the first place, but possibly had another scenario in mind. But at the moment it's a restriction that does not make sense.