Make constructor with min and max number of threads more accessible.
Make constructor with min and max number of threads more accessible.

Summary: In some cases we'd like to initialize all threads during construction.

Reviewed By: yfeldblum

Differential Revision: D34170478

fbshipit-source-id: b010f752856656443896a3a95272a6346a9ea456

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