Fix GoAway message handling
Fix GoAway message handling

Summary:
We were destroying the ProxyDestination::onDown lambda during it's execution,
which was corrupting the ProxyDestination pointer inside of it.
The fix is to grab a copy of the pointer and use it instead.

Reviewed By: jmswen

Differential Revision: D5007008

fbshipit-source-id: aa3c4a048bead8b5c5f1768e85a7702a15b60f8f

facebook/mcrouter,facebook/mcrouter,facebook/mcrouter,facebook/mcrouter