Test ThreadPool more (#1136)
Test ThreadPool more (#1136)

These tests previously helped to reproduce Mutex issue #1124.
wohe/cartographer,wohe/cartographer,googlecartographer/cartographer,larics/cartographer,wohe/cartographer,cartographer-project/cartographer,damonkohler/cartographer,googlecartographer/cartographer,cartographer-project/cartographer,larics/cartographer,larics/cartographer,wohe/cartographer,larics/cartographer,damonkohler/cartographer,damonkohler/cartographer,damonkohler/cartographer,googlecartographer/cartographer,cartographer-project/cartographer,cartographer-project/cartographer,googlecartographer/cartographer