getContextStack perf fix
getContextStack perf fix

Summary: getContextStack has been high on the perf list for a while.  This is an easy fix to save at least half the cost of the allocations

Test Plan: same as D2360988, although much smaller perf win.

Reviewed By: yfeldblum@fb.com

Subscribers: alandau, bmatheny, yfeldblum

FB internal diff: D2361000

Signature: t1:2361000:1440006460:c6a5142f7f094103dcf11aff58410ecd45b46f27

facebook/fbthrift,LinusU/fbthrift,getyourguide/fbthrift,sinjar666/fbthrift,sinjar666/fbthrift,Orvid/fbthrift,Orvid/fbthrift,LinusU/fbthrift,facebook/fbthrift,sinjar666/fbthrift,SergeyMakarenko/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,sinjar666/fbthrift,SergeyMakarenko/fbthrift,facebook/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,LinusU/fbthrift,SergeyMakarenko/fbthrift,Orvid/fbthrift,facebook/fbthrift,SergeyMakarenko/fbthrift,LinusU/fbthrift,LinusU/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,getyourguide/fbthrift,LinusU/fbthrift,LinusU/fbthrift,SergeyMakarenko/fbthrift,facebook/fbthrift,facebook/fbthrift,sinjar666/fbthrift,getyourguide/fbthrift,LinusU/fbthrift,Orvid/fbthrift,SergeyMakarenko/fbthrift,facebook/fbthrift,getyourguide/fbthrift,getyourguide/fbthrift,Orvid/fbthrift,sinjar666/fbthrift,getyourguide/fbthrift,LinusU/fbthrift,Orvid/fbthrift,LinusU/fbthrift,sinjar666/fbthrift,sinjar666/fbthrift,Orvid/fbthrift,facebook/fbthrift,sinjar666/fbthrift,Orvid/fbthrift,Orvid/fbthrift,getyourguide/fbthrift,LinusU/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,Orvid/fbthrift,facebook/fbthrift,Orvid/fbthrift,sinjar666/fbthrift,sinjar666/fbthrift,SergeyMakarenko/fbthrift