Deal with clock uncertainty for transactional readers

Taking care of what Spencer describes in the last slide of
https://www.youtube.com/watch?v=MEAuFgsmND0&feature=youtu.be
and the corresponding entry in the TODO (which describes slightly different semantics),
namely the case in which a reader stumbles upon a write which has a timestamp in the near future.
It's already caught in l168 - will turn that into a panic instead.