Define default num_event_threads config. (#785)
Define default num_event_threads config. (#785)

(This was not covered by tests because they manually set it to 1.)
damonkohler/cartographer,damonkohler/cartographer,wohe/cartographer,larics/cartographer,googlecartographer/cartographer,googlecartographer/cartographer,damonkohler/cartographer,larics/cartographer,cartographer-project/cartographer,larics/cartographer,larics/cartographer,damonkohler/cartographer,wohe/cartographer,googlecartographer/cartographer,cartographer-project/cartographer,cartographer-project/cartographer,wohe/cartographer,cartographer-project/cartographer,googlecartographer/cartographer,wohe/cartographer