Per event template matching runs in parallel,
using N EvaluatorWorkers each holding a copy of the graph,
and executing different templates.
Change-Id: I9d777fde1ea2bc3155c3eca8a30877aedd669081
Depends-On: I2d1e31e386a2576b3a44ec5b8e15dc49d2a474d3
Implements: blueprint parallel-evaluation