Refactor SessionTestHarness to be TypedThriftRequest-friendly
Refactor SessionTestHarness to be TypedThriftRequest-friendly

Summary:Small refactoring of SessionTestHarness which will make it easier to add support
for TypedThriftRequest in a diff stacked on this one.

Reviewed By: pavlo-fb

Differential Revision: D3122241

fb-gh-sync-id: de08af2b8cc7c1277dad024757d5230cee3c09b0
fbshipit-source-id: de08af2b8cc7c1277dad024757d5230cee3c09b0

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