hazptr: Make using a thread pool executor for default domain asynchronous reclamation the default
hazptr: Make using a thread pool executor for default domain asynchronous reclamation the default

Summary: Add a call in folly init to folly::start_hazptr_thread_pool_executor() to make the default hazptr domain use a CPUThreadPoolExecutor by default.

Reviewed By: davidtgoldblatt

Differential Revision: D19107125

fbshipit-source-id: 1cebe0864de14b54349f723061c4c2b5905b105b

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly