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.
You need to be careful in both of these cases that you don't move the timestamp backwards from what it already is. tdb.timestamp must always increase according to reply.Header().Timestamp. That may actually have moved it past MaxTimestamp or past t.ExistingTimestamp and this code could regress it.