Signal on collect on delivery can make the memory consumption explode.

[pstutz]

It's a cool policy for TripleRush, but it has to be customisable, because for PageRank it leads to a nuclear explosion of signals.

Relevant code:
Line 146 in WorkerImplementation:
if (vertex.scoreSignal > signalThreshold) {
//vertexStore.toSignal.put(vertex)
// TODO: Unify scheduling related code. The policy here should be pluggable and the same as
// the one in AkkaWorker.executeOperations.
executeSignalOperationOfVertex(vertex)
}