Add sizeGuess() method to PriorityLifoSemMPMCQueue
Add sizeGuess() method to PriorityLifoSemMPMCQueue

Summary: PriorityLifoSemMPMCQueue only supports size() method. We need sizeGuess() method for stats.

Reviewed By: yfeldblum, ddrcoder

Differential Revision: D3786469

fbshipit-source-id: ec02eefe8b034c2fb863dc513130bc4e655d1c04

facebook/wangle,facebook/wangle,facebook/wangle