Create CurrentDateNode implementing UpdateLeafNode. Ensure we can parse $currentDate to a CurrentDateNode.
- depends on
- 
                    SERVER-28621 Parse BSON update expression into an UpdateNode tree -         
- Closed
 
-         
- is depended on by
- 
                    SERVER-28777 Always parse update expression as an UpdateNode tree when featureCompatibilityVersion=3.6 -         
- Closed
 
-         
- is related to
- 
                    SERVER-28982 $currentDate modifiers should not use LogicalClock as source -         
- Backlog
 
-