add pool_executor in pushmi/examples/pool.h, use it when updating the pushmi benchmark and migrating it to Folly.Benchmark

Summary: The PushmiBenchmark was badly bit-rotted. Update it for the latest pushmi design, migrate it to Folly.Benchmark, and implement it in terms of a new pushmi::pool_executor that is a thin wrapper over a pointer to a folly::CPUThreadPoolExecutor.

Reviewed By: kirkshoop

Differential Revision: D14550589

fbshipit-source-id: d808d6f5795ead421e6ee88439adb9ee652d3146