parallelize reading files
parallelize reading files

Summary:
Faster is better.

This is a behavior-preserving change.

Reviewed By: agampe

Differential Revision: D28401775

fbshipit-source-id: 91be200edb6d6e6824016354a226c5898d4f92ea

facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex