STM: Potentially invalid assertion in STM.c

Summary

In the code for committing a transaction we have this sequence:

   ...
        {
          ACQ_ASSERT(tvar_is_locked(s, trec));
          IF_STM_FG_LOCKS({
            // We have locked the TVar therefore nonatomic addition is sufficient
            NONATOMIC_ADD(&s->num_updates, 1);
          });
          unlock_tvar(cap, trec, s, e -> new_value, true);
        }
        ACQ_ASSERT(!tvar_is_locked(s, trec));

To me this seems like an potential bug. We assert a TVar is unlocked after we unlock it. But it's not obvious why the TVar couldn't have been locked by another thread in between the unlock operation and the assert.

I will leave the assert in for now as it might check some non-obvious undocumented invariant. But I will leave this here in case anyone else actually hits this assertion while nothing is wrong.