Added getShared function to Observer::Snapshot

Summary:
Added a function to Observer::Snapshot that allows the user to get the shared_ptr held in a snapshot. This is to make it easier to integrate Observers with other Api's that expect shared_ptrs.

(Note: this ignores all push blocking failures!)

Reviewed By: andriigrynenko

Differential Revision: D6909245

fbshipit-source-id: 4fe108e04f43a5a250bd7bdc3031994833d73a60

folly/experimental/observer/Observer.h

@@ -91,6 +91,10 @@ class Snapshot {
     return data_.get();
   }
 
+  std::shared_ptr<const T> getShared() const {
+    return data_;
+  }
+
   /**
    * Return the version of the observed object.
    */