When the nodemap "vacuum" it generate a new datafile, with a new unique name and
delete the old one. This confuse the stream clone code and create the same kind
of crash we were seeing in the previous patch.
We build a test case where this happens. The next changeset will contains a fix
We can also notice that we are, wrongfully exchanging undo.* files that the
client will not be able to use. This will be dealt with soon.
nit: s/happen/append/