Add `getTransport` helper in a context in McThirftCallback
Add `getTransport` helper in a context in McThirftCallback

Summary:
We want the access to the underlying transport to be uniform whether we're using Carbon or Thirft - allowing us to template on it easily in D21012853

See D19989318 for the carbon equivalent.

Reviewed By: stuclar

Differential Revision: D19989323

fbshipit-source-id: c69110b97f2905a32f597304d1b718de0e4b5de3

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