rollback

When looking at how the `refresh` mechanism works on stream, it ocurred to me that it seems like a (single level) `rollback` mechanism seems like it would be possible to implement, by:
* pointing readers to the `stale` map,
* dropping the current `read` map,
* cloning the `stale` map, as the new write map,
* abandoning the `oplog`,

I think the `first`/`second` drop seems tricky though, because you would need to perform the `second` drop, for the items in the oplog. (i.e. you want to actually drop the values which are being rolled back, but don't exist in the `stale` map, while not performing the second drop on items in the `stale` map).

There was a bit of musing on uses for `non-collection` evmaps and what they are useful for.
I think the addition of rollback could make them more obviously useful.

This is not really something I need, but _seemed not impossible to implement_