Summary:
simple sugary syrup, best used in conjunction with collect/whenAll

Test Plan: unit

Reviewed By: hans@fb.com

Subscribers: trunkagent, folly-diffs@, jsedgwick, yfeldblum, chalfant, cgthayer

FB internal diff: D1999047

Tasks: 6045623

Signature: t1:1999047:1429570631:62361bb43dc5489fe3d4eb31af404faf8a765402