Hmm, we suspend the current transaction in other places, so this argument isn't convincing enough to me :)

I would expect `remove()` to be shunned because it's going to block the iteration for a long time (relatively speaking), causing a lot of entries to be accumulated in the iterator's queue. But I don't think we really need to spell out why we don't support it ATM, as long as we explain that the user *can* call `cache.remove(k)` while iterating.

I would also put a bigger emphasis on this method ignoring any ongoing transaction.