Add an rvalue overload of Snapshot<T>::getShared()

Summary:
This allows to avoid unnecessary copy of `shared_ptr`.

(Note: this ignores all push blocking failures!)

Reviewed By: andriigrynenko

Differential Revision: D20596972

fbshipit-source-id: 6411ee2f735c1db1984bcd83ceddf80e4ec6f22b

folly/experimental/observer/Observer.h

@@ -92,10 +92,14 @@ class Snapshot {
     return data_.get();
   }
 
-  std::shared_ptr<const T> getShared() const {
+  std::shared_ptr<const T> getShared() const& {
     return data_;
   }
 
+  std::shared_ptr<const T> getShared() && {
+    return std::move(data_);
+  }
+
   /**
    * Return the version of the observed object.
    */