fix folly::NotificationQueue for move-only value types
fix folly::NotificationQueue for move-only value types

Summary:
folly::NotificationQueue::tryConsume did not compile for
move-only types such as folly::Function. This diff fixes that.

Reviewed By: andriigrynenko

Differential Revision: D3711406

fbshipit-source-id: a52eba3eb31743165e4726f830f2a38d241f25a5

Orvid/folly,Orvid/folly,rklabs/folly,floxard/folly,facebook/folly,floxard/folly,floxard/folly,floxard/folly,rklabs/folly,facebook/folly,Orvid/folly,facebook/folly,charsyam/folly,facebook/folly,facebook/folly,charsyam/folly,Orvid/folly,floxard/folly,rklabs/folly,rklabs/folly,charsyam/folly,charsyam/folly,Orvid/folly,rklabs/folly