Fix fbtrace stack usage
Fix fbtrace stack usage

Test Plan: unit tests

Reviewed By: @spalamarchuk

Differential Revision: D1907262
seem-sky/mcrouter,facebook/mcrouter,synecdoche/mcrouter,easyfmxu/mcrouter,evertrue/mcrouter,is00hcw/mcrouter,seem-sky/mcrouter,facebook/mcrouter,evertrue/mcrouter,glensc/mcrouter,apinski-cavium/mcrouter,leitao/mcrouter,glensc/mcrouter,leitao/mcrouter,synecdoche/mcrouter,is00hcw/mcrouter,tempbottle/mcrouter,zhlong73/mcrouter,leitao/mcrouter,yqzhang/mcrouter,leitao/mcrouter,zhlong73/mcrouter,evertrue/mcrouter,yqzhang/mcrouter,facebook/mcrouter,apinski-cavium/mcrouter,easyfmxu/mcrouter,easyfmxu/mcrouter,yqzhang/mcrouter,reddit/mcrouter,glensc/mcrouter,nvaller/mcrouter,reddit/mcrouter,zhlong73/mcrouter,zhlong73/mcrouter,seem-sky/mcrouter,facebook/mcrouter,easyfmxu/mcrouter,nvaller/mcrouter,tempbottle/mcrouter,reddit/mcrouter,nvaller/mcrouter,synecdoche/mcrouter,yqzhang/mcrouter,seem-sky/mcrouter,apinski-cavium/mcrouter,is00hcw/mcrouter,is00hcw/mcrouter,nvaller/mcrouter,evertrue/mcrouter,tempbottle/mcrouter,glensc/mcrouter,reddit/mcrouter,tempbottle/mcrouter,synecdoche/mcrouter,apinski-cavium/mcrouter