AcceptRoutingHandler should not assume a folly::AsyncSocket

Summary:
There is no reason to perform a cast to folly::AsyncSocket, when all of the
required behavior is in folly::AsyncTransportWrapper.

CVE-2019-3554

Reviewed By: avasylev

Differential Revision: D13599212

fbshipit-source-id: 3bb5475fe464c04cf5c04115f34e1bcf237cb4aa