Fix for sendRemoteThread crash on unique pointer release
Fix for sendRemoteThread crash on unique pointer release

Summary:
Get ProxyRequest reference before call to blockingWriteRelaxed. There
is currently a dependency on the order of evaluation of arguments that
means that release on the unique pointer can come before the call to the
function.

Reviewed By: andreimaximov

Differential Revision: D15191194

fbshipit-source-id: 0e4493dda687087a0f25ec940e72a7f0361b95d8

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