Change `JsonNode`, `ObjectNode`, `ArrayNode` method return types for improved chaining

[cowtowncoder]

(background: see f.ex.FasterXML/jackson-databind#1781 -- multiple issues filed against jackson 2.x)

Return values for many methods of JsonNode and its sub-classes (esp. ObjectNode) are sub-optimal, but in practice unchangeable due to backwards-binary-compatibility limitations.
Let's fix these for 3.0.