Handle undefined RUSAGE_THREAD
Handle undefined RUSAGE_THREAD

Summary: Handle cases where RUSAGE_THREAD is undefined.

Test Plan: Unit tests

Reviewed By: @int, @​jmb

Differential Revision: D2310765
yqzhang/mcrouter,is00hcw/mcrouter,easyfmxu/mcrouter,nvaller/mcrouter,reddit/mcrouter,zhlong73/mcrouter,is00hcw/mcrouter,zhlong73/mcrouter,synecdoche/mcrouter,nvaller/mcrouter,facebook/mcrouter,yqzhang/mcrouter,zhlong73/mcrouter,tempbottle/mcrouter,tempbottle/mcrouter,easyfmxu/mcrouter,tempbottle/mcrouter,yqzhang/mcrouter,facebook/mcrouter,facebook/mcrouter,is00hcw/mcrouter,nvaller/mcrouter,synecdoche/mcrouter,reddit/mcrouter,reddit/mcrouter,tempbottle/mcrouter,synecdoche/mcrouter,yqzhang/mcrouter,zhlong73/mcrouter,easyfmxu/mcrouter,reddit/mcrouter,facebook/mcrouter,nvaller/mcrouter,easyfmxu/mcrouter,synecdoche/mcrouter,is00hcw/mcrouter