Pin shadow requests to specific proxy threads

Summary: Shadow requests are pinned to specific threads based on destination host when routed via `SRRoute` and `jump_threads=True`. This greatly reduces the number of connections to the shadow tier.

Reviewed By: stuclar

Differential Revision: D66558771

fbshipit-source-id: b501a126614732de52cf4d16ce004f6872348a14

mcrouter/routes/ShadowRoute-inl.h

@@ -31,6 +31,8 @@ void ShadowRoute<RouterInfo, ShadowPolicy>::dispatchShadowRequest(
         // we don't want to spool shadow requests
         fiber_local<RouterInfo>::clearAsynclogName();
         fiber_local<RouterInfo>::addRequestClass(RequestClass::kShadow);
+        // pin shadow requests to specific threads based on destination
+        fiber_local<RouterInfo>::enableJumpThreads();
         const auto shadowReply = shadow->route(*adjustedReq);
         if (postShadowReplyFn) {
           postShadowReplyFn(shadowReply);