This change allows a Nexter to be used in the same manner as a scanner
using a for graph.Next(it) {} construction.
It is important that graph.Next(it) and any associated it.Result() calls
operate on the same iterator.
We already have reference behaviour, so this is not necessary.
This change highlighted fairly baroque architecture in mql that deserves
some attention; the use of channels is somewhat confusing.
Also rename LastResult to Result.
