Add basic Stream support in `JsonNode`: `valueStream()`, `propertyStream()`, `forEachEntry()`

[cowtowncoder]

(note: part of JSTEP-3 work)

Currently use of Tree Model with Java 8 Streams is bit more complicated than necessary, since there are no java.util.stream.Stream producing methods in JsonNode. But it should be straight-forward at methods as follows:

1. Stream<JsonNode> JsonNode.valueStream(): for ArrayNode return Stream of all element JsonNodes; for ObjectNode return Stream of all values of child entries; for other nodes empty Stream
2. Stream<Map.Entry> JsonNode.propertyStream(): for ObjectNode return Stream of all properties (name, value pairs); for other nodes empty Stream
3. void JsonNode.forEachEntry(BiConsumer<String, JsonNode>): for ObjectNode calls consumer with all entries; for other nodes does nothing.

In future we could also consider some of:

• "parallel" stream variants (for valueStream(), propertyStream())
• "spliterator" for value/property streams

But let's start with the simplest cases first.

[cowtowncoder]

NOTE: renamed entryStream -> propertyStream as per naming guidance of JSTEP-6