Fix race condition in collect(..)

Fix race condition in collect(..)

Summary: This is a temporary fix (until D2015320 is checked in) for race condition(s) in collect(..) method.

Test Plan:
Run unit tests
Run buffalo_aggregator canary

Reviewed By: jsedgwick@fb.com

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

FB internal diff: D2037406

Tasks: 6894157

Signature: t1:2037406:1430435227:ed9612d016cdbd708e2deba02dc4fe0b59632f5a