Add function that enable all observer options for AsyncTransport
Add function that enable all observer options for AsyncTransport

Summary: Enables an observer to automatically subscribe to all available signals.

Differential Revision: D29255979

fbshipit-source-id: 3675ef9bf2442c3b6e26c331a6089f42c1fd8ee9

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly