Add debug string to raylet (#3317)

* initial debug string

* format

* wip debug string

* fix compile

* fix

* update

* finished

* to file

* logs dir

* use temp root

* fix

* override

src/ray/object_manager/object_manager.h

@@ -181,6 +181,11 @@ class ObjectManager : public ObjectManagerInterface {
   /// to this method.
   ProfileTableDataT GetAndResetProfilingInfo();
 
+  /// Returns debug string for class.
+  ///
+  /// \return string.
+  std::string DebugString() const;
+
  private:
   friend class TestObjectManager;