pre-allocate freeList
pre-allocate freeList

Summary: We know ahead of time the size of this list, so may as well pre-allocate space for it

Reviewed By: yfeldblum

Differential Revision: D22838412

fbshipit-source-id: 7d70495fe0910e65dfea49fe7baf675308ab9331

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