Nodes that are scheduled can be executed in any order in theory. So when
there are many scheduled nodes, it can be benefitial to start evaluating
them in parallel.
Note that it is not very common that many nodes are scheduled at the
same time in typical setups because the evaluator uses a depth-first heuristic
to decide in which order to evaluate nodes. It can happen more easily in
generated node trees though.
Also, this change only has an affect in practice if none of the scheduled nodes
uses multi-threading internally, as this would also trigger the user of multiple
threads in the graph executor.
Jacques Lucke
7bd509f73a