Fix ScribeClient singleton leak
Fix ScribeClient singleton leak

Summary:
2 things fixed
- We weren't destroying RouteUpdateLogger
- We were caching shared_ptr<ScribeClient> in RouteUpdateLogger
this combined with not freeing RouteUpdateLogger meant
we leaked the ScribeClient on shut down. This caused
a assert in debug build and a warning in opt build

Differential Revision: D3254989

fbshipit-source-id: 763c6850d589a8141ab41d6fab8133c216a6ef1f

peterlei/fboss,peterlei/fboss,peterlei/fboss,peterlei/fboss