Detached side-effects aren't attached when remoted

This problem is exemplified in gremlin-python:

>>> vFrom = g.V(1).next()
>>> vTo = g.V(6).next()
>>> g.withSideEffect("t",vTo).V(vFrom).as_("f").addE("knows").from_("f").to("t").toList()

the vTo doesn't attach when it reaches the server, so when the addE() gets called you end up with a failure as it tries to add a detached vertex to a graph.